The same reason people have complained about HM moves in general since day one, and cried tears of joy when we realized they won't be a thing in Sun/Moon: it sucks to be forced to have certain moves on your team that you don't wan because you're now limited in how you want to build your team. HM moves mean you're faced with three choices:

1) waste a valuable team slot and have a HM slave along for the ride throughout the entire adventure (I did this in ORAS)

2) constantly go to your PC to bring out a HM slave to do a five-second task (I did this in X)

3) divvy up the HM moves throughout your entire party (Did this when playing the games as a kid)

It's annoying and no one liked it. That said, it's a mark against ALL Pokemon games prior to Sun/Moon, not just Gen III. I loved Hoenn, and Linoone ultimately became a respected member of my ORAS team who solo'd an Elite Four member despite knowing only HM moves. But it admittedly feels more egregious in that region because you need three mandatory water-type moves. And no one wants to teach any Pokemon three moves of the same type.