Interesting spot!
Probably an accidental directorial thing at my end.
We normally do invite based auditions and industry casting calls meaning that every performer has to record a short excerpt for us to hear and decide on but I think that might put certain types of voice actors at a small disadvantage.
Magnus as a franchise tends towards mucky, naturalistic performance but that means that often, voice actors who have spent years on corporate gigs, digital games, animation etc. learning to give pristine soundbites can sound a bit out-of-place. We want mucky, grungy, fidgety, believable delivery with non-verbal reactions when other characters are talking. For any other project those are a bad thing but for us, they are essential.
Experience in live performance does help when it comes to live appearances etc. but that is a secondary consideration. Also lets be real, its rare these days to find a performer who only works in one medium. The money is rarely good enough to be that specific, and I anecdotally believe most professional performers in the UK still get their start via theatre (though happy to be corrected on that if anyone has real data to work from.
TLDR: I think we end up with more theatre or film performers than you'd expect due to Magnus needing less sanitised performances than most other V.O. work.