Because of the reach. The percentage of the population that routinely read is small, really surprisingly small. The percentage that watch broadcast 'television' content (whether that be OTA or streaming or whatever) is high. As we've seen recently there's a bunch of people out there who have the propensity to do literally whatever they see someone else doing on the screen, tide pod challenge, devious licks, you name it, if they see someone else doing it then they'll do it too.
I'm not a censorious person at heart but you do have to be very cognizant about what's being broadcast to your population because it's surprisingly easy to stir up enough people to cause issues, especially if they're already stressed, and Northern Ireland in the 80s was a desperately poor and grim place.
Just interested to know if you're actually contending here that broadcast media has, in general, less of an immediate and direct reach than literature.
Actually, it's worse than that; the SNP is specifically named (as opposed to the "Alba Freedom Party" or somesuch), and the terrorists have (based on the plot on Wikipedia) some fringe but real connection to the party. I presume that the writers had in mind Sinn Fein's role as the legal, public face of the IRA.
I suppose it depends on your definition of 'bad', but the 1970s, were the bloodiest times of the Troubles; 1972 being the worst. The 80s were comparatively 'less violent'. At least in terms of civilian killed.