Not to mention Jeremy Bulloch (who plays Boba Fett) is one of the nicest, fan friendly actors in all of Star Wars.
It all started with his appearance in The Star Wars Holiday Special back in 1978, and the release of his vintage action figure back in 1979. He appeared in a cartoon on The Star Wars Holiday Special, and his vintage action figure was initially available as a mail-away action figure. Many fans were actually disappointed with the character when he appeared in The Empire Strikes Back.
Consequently, many Star Wars fans apparently consider him a must-have autograph.
Speaking strictly to this point, a lot of SW collectors are completionists. There are plenty out there who view "third Stormtrooper from the left" as a must get autograph. Even if you only have 5 seconds of screen time, there are going to be a bunch of people wanting to throw money at you to write your name on their stuff. The number of Lightning Bear, Hal Walmsley, etc. signatures out there speak to this point.
Beyond that, Jeremy Bulloch is among the cheapest of the accessible SW actors. There are Stormtroopers who don't beat his prices. And, as mentioned, he's just an all around nice guy to meet. Contrasted with, say, Peter Mayhew, Jeremy Bulloch is the kind of guy you really don't mind giving your money to.
Tying both together, Fett was in some of the more iconic ESB scenes, such as the Carbon Chamber. So Bulloch being accessible, affordable, and good conversation to boot, combined with the character being individually recognizable from some fairly well known scenes, puts him in the "get" category of a number of collectors.
That's assuming, of course, that you're talking about Bulloch. If you're actually asking about Daniel Logan, then I'm just as confused as you are.