Consistency with previous generations. Its the same reason for the moss rock.
"Just give Eevee a leaf stone!"
"But... its not working."
"Oh, well that's Ruby version. It only works in Diamond Version."
"That's dumb."
"You're dumb."
In truth it's probably not dumb. I'd prefer they just ignore inconsistencies with what could be done in past games and just do what makes more sense now.
Yeah, I would actually prefer it if they retconned new evolutions methods for old pokemon. Pretty much just because there's loads of incenses now, and we'll just keep getting more for new prevos.
HM's
I hate having to waste a precious moveslot on crappy moves like Rock Smash and Rock Climb. Carrying around an HM slave is annoying also. They should definitely use items in Gen VI that you collect over the course of a game to get past boulders or surf waters instead of HMs...
What if they were changed to be slightly better?
Here's How I'd handle it:
Cut: Steel, Physical, 70BP/90Acc, 10% chance to lower opponent's defense one stage.
Fly, Dive, Waterfall and Surf are fine.
Strength: Fighting, Physical, 70BP/90 acc, 10% chance to raise own attack 1 stage.
Rock Smash: Fighting, Physical, 60BP/100acc, removes Stealth Rocks on your side of the field.
Defog: Flying, other, --/--, removes current weather
So there are still better options, but in game you're not at a total handicap.