I can answer that to some degree. It's partly the real-life political things and the social comment in it, even if it mainly sticks to being a "regular" COLUMBO episode.
Of course, there are a few others like that, like IDENTITY CRISIS, A CASE OF IMMUNITY and NOW YOU SEE HIM. But the first one tries to separate the murder from the C.I.A. part to some degree, and the second one is about a FICTIONAL Arab country, which makes SOME difference, and the third one manages to keep the obvious thing in the background (something it's hard to imagine any other story doing with that subject, and I mean that as a good thing).
But in THE CONSPIRATORS, the I.R.A. part of the story is right out in front, and the arms smuggling part gives it a sort of "gritty" crime story feeling. So even though I like it well enough, that's why it somehow doesn't feel entirely like a COLUMBO.

It felt like a Columbo because of all the columboness put into it, but could have been a Kojak tv movie otherwise.