Yes, but when you say a few years ago, that was during the boom year for conventions in general. (Though, I assume you are talking about 2014.) In that case, that was due to the sheer volume of attendees (10,011) and the resulting budget that comes with that many attendees.
Considering the dropping numbers that is trending across all conventions, it's getting a bit risky to reach for more star power overall, lest you wind up running out of funding like other failed conventions.
To step away from the admittedly speculative bit on the convention scene, also consider the actors' prior obligations. Before I begin, another list:
Tara Strong (Twilight Sparkle): Jul 12th 2018, Florida Supercon 2018
Ashleigh Ball (Rainbow Dash, Applejack): working on her solo music post Hey Ocean.
Tabitha St. Germain (Rarity, Princess Luna): She's coming to BronyCon.
Andrea Libman (Pinkie Pie, Fluttershy): unclear- no recent activity found.
We also get Michelle Creber practically every year.
Then, there's the writers and artists that, frankly, don't get nearly as much love. (Except the utterly memetic M.A. Larson)
Remember that actors are people too, with their own lives, wants, and volitions.