Reason #1: Almost everything is a skill. Learning spells is a skill, learning a weapon is a skill, gaining hits or PP is a skill. Not only does this streamline the system, but it allows me to run a game that spans several epochs by simply assigning a technology prereq to each skill (e.g., short sword requires bronze, broad sword requires iron, Lofty Bridge requires closed essence, etc.). In RM2 there were separate mechanics for spells, weapons (with a level bonus), hits, power points, etc. (And don't get me started on D&D 3.5 with skills, weapons, spells, feats, etc.!)
Reason #2: Every skill has a discrete category. Thus, adding new professions is simple. Just assign costs to the fixed categories, add some Profession bonuses, and you're done. This feature also makes it easier for me to put everything in a database.
Reason #3: Spells were streamlined, and spell types explained. In RMSS I never have to have the argument regarding Long Door-ing an opponent 100' into the air.
Reason #4: Stats were streamlined: Most players seem to prefer a point-buy system (I like random stats, without re-ordering, but I know I'm in the minority). Dev points are calculated (no table needed), potentials can be calculated (there's a table, but it's formulaic), and stat gains require no table.
Things I don't like about RMSS:
* Training Packages require separate costs for every profession. And, some costs simply aren't available. TPs should rely on a fixed formula instead.
* A handful of mechanics aren't skill-based: initiative, RRs, exhaustion points. I found it easy to make Endurance a Body Development skill to handle exhaustion points, but there isn't a simple fix for the other two.