If that were the case, why don't all runways that accept aircraft with long wheel bases have same?? (See other thread re pilot eye height)
KISS:
I'd say that's because v.few aircraft require a 1500ft aimpoint, and to have a 1500ft vasis would work against the vast majority of aircraft normally using a 1000ft aimpoint.
Furthermore, most of the airports which Qantas flies the 400 into have a 1000ft vasis. Obviously the 1000ft vasis is acceptable to 737 sized aircraft which are still the greatest users of these same airports. So a normal 1500ft aim point requires 2 dots fly down on the vasis.
WG.