Why don’t a lot of themes on TF use frameworks like Gantry? I’ve only noticed a handful of themes based on a framework.
My reasons and the ones I hear repeated often are these (with regards to Gantry, Warp, etc):
1) (Feature) Heavy footprint – Memory requirements / performance is immediately impacted.
2) Not responsive / mobile friendly
3) Unless you don’t care what’s in your theme, it can be complicated to understand such extensive frameworks. You can easily put your potential clients / theme buyers at risk from unknown code / security exploits. You may also open yourself up liability lawsuits (deserved or not).
4) Too many lighter, open source frameworks of equal or superior code quality are available.
5) Many theme authors make their own framework so they don’t have to rely on a 3rd party to address security / code quality issues, etc.