IDK exactly who was sponsor prize money, but it may be that Chevron was more willing to put up money in Houston than Asics is in LA.
Chevron probably has a couple of hundred billion more in revenue than Asics. $100,000 is chump change for Chevron and they have been very generous with the Houston Marathon. Asics probably includes LA in a comparatively more limited US sponsorship/advertising budget.
Hopefully, someone in LA will step up to improve the prize money. There's obviously no shortage of major corporations there for the LA committee to hit up.
As for the decision, Siegel wanted LA but so apparently did the USOC and NBC so it may not be fair to put it all on him.