I feel like they don't do the fusions too often is for one of two reasons: 1) if the fusions happened very often they might come off as seeming "less special", or 2) since all fusions are voiced by singers, they probably can't get all the singers needed as often because of scheduling problems.

Of course Estelle as Garnet is the one exception, but I feel certain Sugilite is one we may not see too often because Nicki Minaj is a a pretty big singer right now and scheduling her for voice recordings is probably not very convenient for Cartoon Network (notice the last time she appeared she had no speaking lines).

[For anyone curious about who Sardonyx's voice is, it's Alexia Khadime, an English play actress who's been in musicals like Wicked, Les Miserable, and The Book of Mormon.]

It's a shame Opel's actress is so hard to get, since her voice is beautiful. Honestly, I want another Opel episode just so we can hear her talk again.