The real reason for southern hostility to SF?

Jude Collins criticises the trail of senior representatives from the Republic coming in to back the SDLP. He believes the real reason for their sympathies is that parties political inertness, and inability to touch their political hinterlands in the Republic:

The mistake in trying to follow the logic of southern government thinking is to focus on who they appear to be pro. The truth is, they’re not pro anybody, other than themselves. If you want to reconcile these three-different-directions-at-once stances, ask yourself: “Who are Bertie and Michael anti?” You got it in one. Sinn Féin.

And sure why wouldn’t they be? If you were in their shoes, you’d be the same. It’s not the SDLP that’s going to come down and take votes from Fianna Fáil. The SDLP is going nowhere and taking votes from no one. And the DUP or the UUP aren’t going to head south to start rearranging the Dáil’s political furniture.