I think there's a sampling bias going on here. I think "I don't have enough space" could indeed be the most important reason... if you sample members of Photrio who do not print at home.
For me, the barrier to entry was not lack of space, but the intimidating cost and size of this weird device called an "enlarger", that apparently I am supposed to buy second hand from Craigslist or whatnot, in lord knows what condition, without knowing if I will get it to work, just to attempt a new hobby that I might not even enjoy.
EDIT / PS: Even now, the main barrier for me trying medium format is not lack of interest, but lack of an enlarger that can handle medium format.