First, Smogon rankings have little to do with the "main" Pokemon games. Smogon rankings don't apply there, especially in casual play (casual as in non-competitive). Second, since when Slowking is OU? According to Smogon Slowking is NU, see for yourself: http://www.smogon.com/dex/sm/pokemon/slowking/
edit: to clarify: Slowking appears to be ranked in NU, but has its uses in OU
Last, Alolan forms aren't necessarily the same level of boost in power as Mega Evolution is. Just take a look at A- Exeggutor and A- Dugtrio (Smogon rankings) and compare them to their original counterparts. You do have a point when you say that the mons nobody uses should get Alolan forms. Since you brought up Mega Evolutions as comparison, allow me to do the same. Audino from what I understand is still ignored even though it got a Mega Evolution. Implying the forgotten mons are used more should they get new Alolan forms comes across as (misplaced) optimism. I guess we have to wait and see.
I still prefer Water/Steel Alolan Kingler, though. Definitely a less used mon that could use some love, but you know, since it happens to be in wrong gen...