Because, as I can tell you from somewhat limited experience, being an infiltrator is a very situational class. There are a few cases where it's incredibly useful, and the rest of the time it's either useless or downright detrimental.
And even when there is a situation where an infiltrator can make his mark, it requires a load of training - which I only found out about after Rosnan (the infiltrator among my characters)... well, epically failed at an attempt at stabbing someone.
He got slapped with a fine from the judge of his realm (I probably should have checked diplomacy settings before trying to kill foreign government members) which he had no hope of repaying. 200 gold doesn't sound like a lot, but this was post-blight Beluaterra, where land productivity was so dismally low that a region lord in, say, Enweil could optimistically hope for maybe fifty gold on tax day. The target was chosen solely because he had the highest bounty on his head of anyone remotely nearby. It was a monetary issue.
In fact, that's the only reason I made him an infiltrator - because as I found out one good raid on a foreign tax office could bring in more cash than a whole week of waiting on the estate. I later realized how reckless this was, but it's still incredibly safe compared with assassination attempts. I think that's part of it - infiltrators take huge amounts of training to hurt people, but almost none to steal money, which makes them more of glorified thieves unless you grind 'em at the academy sixteen hours a day with the expert tutor. Which, again, costs more money than Rosnan ever had. Vicious cycle.
So, to wrap up my obnoxious narrative attempting to justify playing my first character like a jerk, this is all a roundabout way of saying infilitrator training is a pain in the neck. I think that's one reason they're unpopular. They're just hard to play, and require a lot of dedication for comparatively little payoff. The only place where there's anything to gain other than money or a stay in the dungeon is when you're going "in the service of the realm", and in that case it's safer to just get a big unit and go to battle anyway. And the fact that most realms RP being above that, regardless of its efficacy, which can't be controlled.
I guess one thing is that if anything the infiltrator deserves to be sorta kinda very slightly just a tiny bit overpowered in terms of mechanics to make up for the fact that in most places they're completely anathema in RP situations and realm laws.
