I attribute it towards better design and SQ expectations that doesn't correlate with 6x9 speakers. 6x9's do not have the capability to reproduce sounds accurately due to their physical dimension.
Also, America's SQ/Audiophile companies are pretty much gone. Go check out SQ oriented head units. Alpine has one, Pioneer has one, Clarion stopped making theirs and Blaupunkt MIGHT release a north american version. That leaves Nakamichi and another I can't remember, both imported. The latter two don't have processing capacities needed for an SQ oriented installation.
If you're looking at home markets, the very high end is imported from Europe and Asian markets. That leaves the DIY base. I can't say I've seen any installations using 6x9's on DIYMA.com or car stereo shops building for sq competitions.
Maybe some of those companies CAN make good and accurate 6x9's but I've never heard them :/
I think our markets are falling apart. (sigh).