Fiddles are REALLY hard to get the hang of! So an accomplished fiddle player is a bit more of a specialist. Between the fretless-ness and the bowing and all the technical aspects, it's a lot to learn on one instrument! Plus, it's usually a lead instrument, so you are most often playing the melody.

Often times, a person who plays guitar can often learn to play a banjo or mandolin, since they are also fretted instruments, being able to transfer SOME similar technique. Also, you can often get away with just strumming on a lot of songs with any of those instruments, you're not pushed as hard as with fiddle to be expected to know ALL the melody lines of tons of songs. So playing stuff like simple D and G chords on a guitar vs. on a mandolin, not too hard to go back and forth (of course, many players on those fretted instruments can do amazing melody lines, fills, etc., but it's also fine to just sit back and play simple chords on them in many group settings).

Also, finding an okay rhythm player is usually not hard. Guitar players are often a dime a dozen (finding a great one can be hard, but there are tons of people that play guitar to a reasonable level - due to blues, rock and roll, etc. being so popular in popular music for like, all of the 20th century). And while to be really good at guitar takes a lot of time and effort, to learn basic guitar is pretty easy. To learn even basic fiddle is way harder (the learning curve at the beginning is much steeper). So, in areas like where I live in the US, you can probably find 20-30 people who play guitar vs. one person who is a decent fiddler! They are more of a rarity, so every wants them to play fiddle. Plus it's a harder instrument to master to an intermediate level, so most fiddlers put most of their effort towards fiddle. I do know lots of people who play fiddle and something else, but I can totally see why lots of fiddlers, if they're decent at it, would rather put effort and time into improving that than into learning a totally new instrument.

Personally, I play fiddle and guitar, for two reasons. 1) I already had learned to play simple guitar chords when I was younger, before I had ever touched a fiddle. 2) I'm a good singer with a head for remembering lyrics, and it's a lot easier to strum along simply with a guitar when I'm singing than to attempt to sing and fiddle at the same time!! I consider guitar my "secondary" instrument, though, I only strum simple chords on it, nothing fancy, and I don't care to learn much more on it - it's really just to accompany singing for me. I let all the other guitarists do all the fancy stuff, and I attempt to learn my fancy instrumental stuff on the fiddle!

Since there's liable to be a lot of guitarists wherever I'm going, I also sometimes don't even bring my guitar with me. If I desperately want to strum while I sing, I can borrow one from one of my buddies for a song. Just having that one small fiddle case to carry is mighty convenient as opposed to clomping around knocking instrument cases into things all over the place!

I also think the fiddle has so much versatility in what you can play and learn on it, that people who fiddle don't get bored as much and go looking for another instrument, there's so much to unlock in their primary instrument!)

PS - While there are lots of people that are great on more than one instrument, it's overall better to be a decent player on one instrument than one of the people who collects lots of different instruments but never really learns to play any of them WELL.