On the contrary, 2x10 bike setups are getting more common these days even for MTB, all the manufacturers offer 2x10 setups.
One big advantage is that all combinations on a 2x10 are usable, whereas with a classic 3x9 you have to avoid being at high gear up front and low gear at the back, or you will be crossing the chain and damaging components / dropping chain.
I run a 1x10, and there is no problem with chain crossing. The 10/11/12 speed cassettes are basically not any wider than the regular 9 speed ones, so there is no more chain crossing than what you get for the middle ring on a 3x9. The optimization is all in narrower chains and sprockets.
That being said, I do run an upper chain guide to reduce probability of the chain falling off the chainring when the rear derailleur gets clogged with mud etc.
One big advantage is that all combinations on a 2x10 are usable, whereas with a classic 3x9 you have to avoid being at high gear up front and low gear at the back, or you will be crossing the chain and damaging components / dropping chain.