I think it's mostly because how good the Radiance can be in early confrontations rather than the Revelation being all that bad.

The Radiance gets Detonate Anti-Matter which is useful early, but also Energy Absorptive Armor, which being a passive ability is very nice to have since it not only helps armor but feeds the Anti-Matter pool. Combine that with Animosity and it can be pretty good at drawing fire. If the Radiance survives to Level 6 too, it gets the Cleansing Brilliance, which is just a great ability, especially when you have Motherships using Malice. The Radiance as a whole if you're going to run into an enemy early is a strong ship to have, and later in game having a few helps.

The Revelation from my experience is a pretty good ship, but only in limited numbers. It's ability to disable an enemy ship is pretty good, but the Revelation doesn't have the as good a punch to take it out. As it levels it doesn't get quite as effective as a Radiance, since it doesn't have the same Armor bonuses coming in, Guidance and Clairvoyance aren't bad abilities, but aren't quite as useful as the Radiances in many direct fights. Although if you can get it to Level 6, Provoke Hysteria is another wonderful ability in it's own right. Having a Revelation clear systems though early on a big map doesn't sound half bad though, busting out Provoke Hysteria early would probably one mean surprise.

Personally I prefer either a Radiance or a Progenitor for my first ship. The Radiance for what's above, it's a good toe-to-toe fighter, and the Progenitor since it has a pretty much unrivaled Shield Restore as well as Malice and Colonize. I only really use Halycon's and Revelations later in game and fairly rarely use the Rapture.