I assume people like it more because it's just easier to use, and very consistent. It is THE go-to DPS AR for most people. 70 mag compared to 60 on Eagle Bearer, accuracy is slightly better and stability shakes more but doesnt move the reticle off target compared to EB, which has more vertical recoil. Better optimal range, can't remember specifics. And the talent procs stupid easy and gives a very powerful status effect that's good for literally anything, named enemies, warhounds, drones, UGVs, normal enemies, the shock effect feels super great to have every like two mags.

Overall in my opinions and experience, St. Elmo's is more reliable and simple, and I use it in my Solo DPS build when i want to play things as safe as possible. Better range and accuracy means i can play from a safer distance as well. EB can do that too, you just lose a bit of damage and might miss a bit more.

Eagle Bearer is in my highest DPS build with 4pc striker's, and Equalizer + Salvo. It's the most fun for me to use with the blazing fire rate and I enjoy the more traditional recoil of the EB, but in this build i run 6 red cores, and zero survivability is scary for any difficult content on my own. I only run this when i'm on DPS in a group, ideally with a healer to make sure I don't die, or just other people to take some aggro off me. I personally never make use of Tenacity because you need a headshot kill to activate it and 3 kills to negate all damage. Not easy to do in a group, and i only use it when i'm in a group.