I assume you meant to say "I don't see why it isn't better," otherwise your whole post doesn't make sense. The reason is because Return will let you switch out and Hyper Beam will leave Slaking in there, useless until the next turn which will let a whole host of sweepers setup on you.
The only time I'd use Hyper Beam on Slaking is if the rest of my Pokemon have no trouble with Skarmory, in which case I'd drop Focus Punch. But even then, I prefer Fury Swipes, so Slaking suddenly becomes a non-protect Ninjask counter, not vice versa. Multi-hit moves are also nice to surprise Pokemon that think they're safe behind a sub against Slaking. The accuracy is a bit of a let down, though. :(
The choice between Double-Edge and Return is a far harder one, imo.
HB can what? OHKO Weezing and do a bit more to Suicune? It's not really necessary. Even if you do predict properly, you're just giving your opponent 1 or 2 100% free moves. Say you Hyper Beam as they switch to Milotic and you OHKO it. Your opponent than sends out Gengar, WoW's your Slaking and Focus Punches whatever you send in next turn. Pretty gay.