I think it's more the parochial nature of the north (or at least certain communities up there anyway).
It wasn't until the last couple of years that IL clubs started taking European football seriously by not letting key players and even managers go on holiday when the games are on. The Setanta cup never really captured the attention of IL clubs even though 2 of them won it. If it's not local, and especially if they'll struggle to win, then they just aren't interested.
Sure fans of Belfast clubs used to complain about having to travel an hour and a half to games in Ballinamallard. Small minds that are happy in a small league. The 'our wee country' outlook. It's why an all-island league will always struggle to get support up there.