I believe it's a combination of both, your theory is valid tho and phill has pointed that out a while back, altcoins mining gears make a lot more money than BTC mining gears when power is the only factor, but this can't be the only reason, because even the manufacturers that are not known to be making altcoins mining gears and focused mainly on BTC gears aren't making enough gears.
It is both, the question is whichever is greater problem, but regardless, this means we will probably be looking at a very low change in diff in next year or so, IMHO.
P.S. My post got deleted, I dont know why, I'm new here but I'm open for guidance on what I did wrong.