Rockstar probably haven't refused to put it on Switch. I mean, they can't refuse to do it until Nintendo actually asks them to do it.
As for why they haven't released it on Switch anyway yet, well is there a market for it? And I don't mean for GTAV itself, but for the microstransactions that come with it? That's how Rockstar are making their money these days. If there's no market for microtransactions on Switch then surely it makes more sense to create a bunch of new stuff to peddle to existing owners rather than to port it to a whole new console.