Selling bikes in Thailand or anywhere is about profits - bet you'd never guessed that!
With those low numbers of existing big bikes in TH there's no reason for the the BIG FOUR to offer new 750s, 900s, 1100s or even bigger choppers, which sell like hotcakes anywhere else, because they won't make any profit that way.
Honda tested the waters with the CBR150R and the 200 Phantom (I'm not sure if Kawasaki came out first with their 175 Boss) - and there are rumors that the CBR will be discontinued. Yamaha, Suzuki and Kawasaki stayed away - no FZR 150 (even though there's a brand new FZR125 for the europeans), no Ninja 150 or 175 to best Honda. They looked at the numbers and decided not to bring in any small sport bikes.
Kawasaki has the 250 Ninja but it's not available here, for above reasons.
So if there's no interest in small sport bikes, why should they bring in a big one, leave alone several models to choose from?