Bigger front wheel rolls over stuff easier, smaller back is more nimble. Along those lines. Oh and fashion probably plays a part too.
plus on longer travel bikes and shorter riders, less chance of catching your arse.
Liteville tried it for years and were pretty much ignored. Then they became race legal* not too long after 29ers became popular in downhill and enduro racing.
*side note, apparently the Mojo team did it earlier, having got their hands on the first gen 29er downhill forks, but not able to get new frames so kept running the 27.5 back end; didnt bother to read the rules, and nobody checked either.