Maybe... look at the fees on each. If they're the same then I can't think of a reason to consolidate (or to not consolidate). If they're different then that's the reason.
I've just moved mine from Aviva to Vanguard for the slightly lower fees.
The two old ones are at 0.35% - nothing being paid in. Total value is less than £20k
Current employer one is 0.4% and is being paid into. Total value is just over £60k
Not loads, but also I'd imagine it's very quick to do if it's easier to see things in a single place.
The Aviva -> Vanguard transfer was all done online without any faff. About 5mins to get the various reference numbers and then it transferred over in 2 days.
Maybe... look at the fees on each. If they're the same then I can't think of a reason to consolidate (or to not consolidate). If they're different then that's the reason.
I've just moved mine from Aviva to Vanguard for the slightly lower fees.