Gary Fisher and To a lesser extent Spesh are really responsible for hammering away at it until the market slowly responded. The whole idea for 29" MTVs goes back to the 70s and Geoff Apps and the Cleland and Highpath brands. He got Gary Fisher interested but the couldn't get a reliable supply of 29" off road tyres in the 70s so mass production was impossible. If course, all this pre dates hybrids and HTs by a very good number of years.
I think a 29 HT for fair condition trail riding is next on my list, but ill need to spend £800+ to get sonething properly useable with wheels that don't flex. Ill keep my 26'ers for wetter, muckier rides, with their smaller wheels so much less probe to clagging and weighing less when they do. Conversely, the drier climate of the SW United States is where the 29 comes into its own, and as tyre supply ever so slowly increased Fisher, and later Spesh were able to offer the production concept that Fisher had dearly wanted to in the 70's, but couldn't. Ultimately we have Geoff Apps to thank for all this, and arguably for making the first dedicated MTB, as opposed to jazzing up a beach cruiser frame as the Yanks were doing at the time.
But in a nutshell, your assertion was wrong. 29" wheeled MTBs go back 40 years to Apps, and had the tyres been available then it would all be very old news.
If course, 650B is an even older standard and on paper it looks like the one that might finally supersede the 26 in muddier climates. Ridden plenty of 29s and really would like to try a 650B (or a 27 as they're now being called) to see if they really do hit that sweet spit with some of the advantages and less of the draw backs. We'll see.
