The IRA and the PIRA were fighting a propaganda war as much as anything else, and assassinating other Irishmen who weren't directly involved in the occupation may have been seen as bad press for the movement. Remember also that a core part of their platform was that the British were an illegitimate force of occupation, so attacking them (and the RUC, which they considered a paramilitary tool of the occupiers) was a legitimate act of warfare. Killing political opponents, on the other hand, would have delegitimized their struggle and made it harder for them to argue against British and Loyalist claims that they were just terrorists. Lastly, although this is more conjecture on my part, the PIRA may have been worried about tit-for-tat attacks on their own leadership if they went after people like Paisley. This would not have been an unreasonable fear, given that much of the violence of the Troubles took the form of tit-for-tat attacks and constant revenge attack feedback loops.

There were several attempts to kill Paisley though I'm not certain how many were by the Provisionals. The IRA did kill several unionist politicians (for example Edgar Graham) as did the Official IRA and the INLA. Contrary to what some commentators have implied the Provisional IRA regularly killed Irish civilians both Protestant and Catholic. Indeed they killed more civilians than any other organisation in the troubles. This was one of the big differences between PIRA and the Republicans described by McGarry (who is great but doesn't deal with the Post 1970 IRA). The best book on the Provisional IRA is Richard English, Armed Struggle. I'd start there.