Because Sega's not known for making the best decisions when it comes to Sonic. That and Sega seems very averse to making direct sequels these days. Save for Sonic 4 which was back in, what 2010, they tend not to make numbered sequels. Hell Adventure 2 is the only 3D sequel as is. I suppose a direct "Mania 2" might not be the best idea if only because it implies a likelihood of remade zones from the past which... we're good. But I hope they do something anyway with the Classic cast. Isn't it odd that after being virtually dead for 25 years, Mighty and Ray are playable while virtually none of the modern cast has been playable in a mainline game since 2006. They went from dead to being more important than half the cast like that
Sega always follows up on successful formulas. SA1 led to SA2 and Sonic 06. Unleashed's boost levels led to Colors, Gens and Forces. Rush led to Rush Adventure, Colors 3DS and Gens 3DS. It would be unlike Sega to drop Mania's classic-style formula after it produced the most successful Sonic game in many years.
Eh, just because it was the precedence set by the 1st one. Then again, SA2 is pretty significantly different than SA1, so I suppose the same could apply here. But Sonic Team has a bad habit of learning the wrong lessons from each game, so God knows what they'd want a Mania 2 to look like (even if they don't develop it their influence is still there, even if it's in the background).
Which is why we need to make it very clear to SEGA exactly what we want/mean when we say "Mania 2", because otherwise a game with more reimagined stages is probably what we'll get. For me a Mania 2 would mean all new stages, a good story like 3&K but with Mania's quality cutscenes/animations. Some new creative level design & gimmicks, bring back Encore mode and do more with it. Make Amy Rose a playable character. Obviously made by Evening Star and Stealth. I'd actually like Sonic Team to have a go at making a new 2d game. Not that I'd expect anything particularly good, but surely they've learned from/can make something better than Sonic 4.
I wouldn't mind Mania 2 reusing zones if it flipped the ratio. Give us four recycled zones that didn't make the cut in Mania 1 and eight brand new ones. Maybe they could dip into some of the Genesis-era spinoffs too - Spinball, 3D Blast, Chaotix.
Those are the exact two zones I would hope for. Not sure what zone from 3D Blast would be worthy of the Mania treatment, all of its levels always felt kind of like value brand versions of more popular Sonic level tropes, but hey, Mania spun gold from a few of the lesser zones.
I'd be fine with level reuse if the levels didn't appear in Sonic 1, 2, 3K, or CD (with the exception of Quartz Quadrant, it's a unique trope that's not visited a lot. Plus the music is fantastic). Any fully realized stage from Chaotix, a Spinball level modeled more after traditional level layouts, that sunset train level from Triple Trouble, stuff like that would be great. The only 3D Blast level I'd like to see is maybe Rusty Ruin. The rest of the levels are all tropes done better in other games in the series. Also I was low key hoping that in the Plus DLC, Mighty and Ray's intro was set on Isolated Island rather than flying into Angel Island on the tornado like Sonic & Tails
I've mentioned this before but I would have loved a "Veg-o-Fortress" zone comprised of the 4 stages from Spinball made into a "proper" Sonic level to fit the pinball trope. The tasks from Spinball could be where the Special Stage rings and 1Ups are located.
That would actually make a really cool level. Something like the Egg Rocket from Sonic Advance 1 but with more variety.
I'd be all for some Chaotix levels, but Spinball? Meh. I've always found its stages pretty dull in terms of visuals and level themes, except maybe for Showdown. What do people find so interesting about them? (Not a complaint, but an honest question)
For me anyway I think the levels have untapped potential with somewhat unique tropes. Mania kept it to the games that shared similar art styles, but Spinball doesn't share that art style. I want to see what they can do to with it to bring it in line with the other games. So basically, it's different Chaotix would need some adjustments too, it went a little too crazy with the visuals (and all the backgrounds were like blurry for some reason)
I would totally 100% love to see some 8-Bit / Game Gear / Master System stages, honestly (and that's a good excuse to hear some new remixes too).
Conjuring a list of stages the community here wouldn't mind seeing return in a Mania sequel could be fruitful. So I'll start: Sonic 1: Marble zone. While Star Light and Spring yard are fun, their layouts and badniks aren't as exciting as their counterparts in later games. Sonic 2: Any scrapped level honestly. Hidden Palace would be cool, but not as cool as others if it didn't hold plot significace. I'd really love to see Woods or Ocean Wind be fully realised. It'd be fun to see Dust Hill replace the GHZ/EHZ starting zone as a ravaged version akin to AIZ' act 2 in S3. Sonic 3: Launch Base or bust. Sonic & Knuckles: Mushroom Hill, simply because I adore it. It has a lot of gimmicks like the elevators and wind-chickens that could be put to good use. Sonic 3D Blast: Rusty Ruin or possibly Spring Stadium to fill the casino/pinball trope. Rusty Ruin would be my 2nd pick after Marble for the ruins trope. Chaotix. Marina Madness or Techno Tower. Mainly for the music in each, but Marina also has water going for it. I felt like Mania was a little too dry overall. Spinball: Toxic Caves. Choosing only 4 of these, I'd go for Mushroom Hill, Marble, Marina Madness, and Toxic Caves. Oh, and I intentionally bypassed CD as I felt that Mania already used the best stages.
Star Light, Mystic Cave, Carnival Night (prototype music to get past rights BS), Mushroom Hill, either Rusty Ruin or Volcano Valley. I would've said Toxic Caves but Mystic Cave covers that archetype, so how about The Machine? Doubles as a reference to Cyber City. Chaotix I'm not sure what to pick. Marina Madness but with water added?
While we're all dreaming up what we'd like in a Mania 2, I'll just say Amy & Ristar side quest with a hot air balloon-themed Zone and leave it at that.
For Sonic Mania 2 they might be thinking: "if we are making 3D Sonic games with a bit of 2D, as in Forces, why don't we make a 2D Sonic game with a bit of 3D?" And then Sonic Colors 2 is born.