demand is the main reason, most of the buyers they have dont own the b18a they own the b18b etc....its all about marketing to the people with the most money, ie the teenagers that just got their first integras that are b18b's....now besides that you can answer your own question on what the changes would be, what are the differences between the b18b and the b18a in engines and engine bays....obviously the piping would be somewhat different (on a turbo set up) bc the engine bay is different, the intake manifolds are different, (b18b has a better one) but the b18a has 9.2 compression which is good for newbie boosting...there are other minor differences that you could check with the manufactors about, but i do know that drag makes a kit for the 90-93 integra b18a, but you have to preorder it bc they make them on demand....hope that helps