I agree. The lack of decent, scheduled bus service - BIG bus service - all the way along between Bangkok, Chonburi, Bangsaen, Sri Racha, Pattaya, and Chanthaburi and Trat is a disgrace.
But the "why" of it is dangerous to speculate on - for our own sanity. My guess is the public sector which could implement a bus route (The Transport Co.?) - and which can do little right anyway - likes the tax revenue from petrol sales to the private vehicles. And my impression of the private sector bus lines is they also are too wrapped up in corruption, fear, and incompetence to make this obvious route successful.