I think it has a lot of potential answers on a case by case basis.
Often people don't want to maintain an old code path. Sometimes people get into that line of thinking without clear understanding of just how many people like what the old code path is doing for them and why. Sometimes they also over-estimate the maintenance cost of the old thing.
In the case of win8, there was a strong drive for "we're doing a tablet now", very little intuition about how their thing was going to be received in the market, and the company culture and talent pool was already markedly different from the one that produced their 1990s greatest hits, so it's not like they were automatically going to reproduce prior success.
Also notable that in vista and win7, turning on the classic look disables dwm.exe, the compositing engine, and some GPU-based optimizations.
Often people don't want to maintain an old code path. Sometimes people get into that line of thinking without clear understanding of just how many people like what the old code path is doing for them and why. Sometimes they also over-estimate the maintenance cost of the old thing.
In the case of win8, there was a strong drive for "we're doing a tablet now", very little intuition about how their thing was going to be received in the market, and the company culture and talent pool was already markedly different from the one that produced their 1990s greatest hits, so it's not like they were automatically going to reproduce prior success.
Also notable that in vista and win7, turning on the classic look disables dwm.exe, the compositing engine, and some GPU-based optimizations.