These r not excuses bhai but plain facts.
Answer to the 1st part of ur post is that companies conduct market research / surveys for various regional markets at various points of time to know if there is some scope to grow its business there and this includes knowing what people like watching, if it is made available or not and incase there is some shortcoming then can they fill the space in a way that it is financially viable move
Answer to ur 2nd point is in ur reply itself. All the OTT platforms u mentioned r primarily catering to respective regional audiences hence they acquire lot of such content so u can subscribe to them rather than hoping to see all content on major national OTT platforms.
Infact zee5, disney+hotstar, sony luv, voot also produce regional web series too, u can watch and review them on OnlyTech forum