Grants from the deep pockets of the fat cats in the barefoot/minimalist/antishoe-industrial complex? Studies on things that are not regulated by government are generally funded by private sources. The only published research on running shoes one is apt to find is research that supports the current design trend (try to hunt down the research on cell phone safety funded by Motorola and the scientist whose said research they refused to publish). Shoe companies are not hiring people to develop designs that go against their own established market.
There is not even proof that there have really been “improvements” in shoe design. Change != improvement.
A scientific study is not needed to disprove something that was never proven via scientific study. People believe a lot of irrational things, often completely in defiance of science and logic. Basically people believe it because that is a perception they arrived at independently or they rather unquestioningly swallow something from a source they believe is authoritative because it sounds true or confirms their own suspicions.
A significant part of what influences people to rush to become competitive runners (or even runners, period) is in effect a marketing campaign by those who profit from the sport to lower levels of acceptance and prudence.