From what I heard, it was due to them starting with the mech's artwork and clicky stats, then having to backfill in Battletech stats based on them, so they had a lot of mechs (and vehicles) with a bunch of guns and the Ballistic damage type. That meant they were stuck figuring out what they could fit two of in a single side torso for a mech that had that speed and mass.
So... that's partly true, but it's a little more complicated than that.
Unique Mechs in the early few sets of MW:DA (everything up to and including the Falcon's Prey release) had a little manila folder included that told you about the Mech and the pilot, sort of a miniature TRO/bio to flesh out what you had in your sweaty little palms. While those didn't include record sheets, of course, since this wasn't intended to be part of newly-rechristened Classic Battletech, it did give info on the Mech outside of that- name, ground speed, jumping, weaponry, manufacturer, that kind of thing.
In the case of the Ryoken II, which was actually from the very first MW:DA release, the dossier did indeed give stats for four LB-2X and two LRM-15. So in a case like that, there's clear stats to work from outside of the basic look of the Mech- and to their credit, the people who made Record Sheets: Dark Age did a pretty good job of taking those stats and making passable units out of them for Mechs like the Ryoken II, Arbalest, Atlas, etc.- and presumably later dossiers helped to flesh out units ranging from the Tian-Zong to the Gyrfalcon as sets continued to release.
Exceptions abound, naturally, since none of this was ever intended to convert to Battletech stats. The Shadow Cat II's dossier, if you build the Mech exactly as the dossier suggests, ends up with 1.5 tons of armor total as I recall. And of course, the Sun Cobra had the odd situation in which one dossier gave it twin Gauss rifles and a Clan background, another had twin PPCs and was Davion-made (and kudos to the authors for finding a way to make that work!). But at least in the case of the early releases, the dossiers were kept as close as possible for conversion in RS:MWDA to give us the Mechs in that book, rather than just saying 'those look like medium lasers, cool'. (Anything post-Falcon's Prey didn't have these handy dossiers, and definitely had to be reverse-engineered by looks alone)
