Your personal opinions aside, Monopoly is a successful games, as it has been around for a very long time and is still selling every year and being played everyday (somewhere). The fact that just about anyone can play it will minimal instruction proves my point that easier to learn games* are more broadly appealing and will, hence, be more successful economically.
I agree that the spellcasting aspect of HARP is more complicated than RM, what with the scaling options and scaling modifiers, but that is really the only part of that game that I have found to be so. Even then, I prefer it to the spell list system, which is something I was constantly battling with - just have never really cared for it much. Some would argue that the list system is more complicated in the way you have to keep track of all the different lists you have to what level you have them at. Sort of problematic also when you constantly have to deal with cross list spells that are the same, yet not. Keeping track of all of that can be confusing, I imagine.
I can't comment on the "designer's end" as I wasn't in the room when they made the decisions to go one way or the other with a particular rule. Who knows, maybe they flipped coins to determine what way to go. Maybe they used quantum mathematics. I don't know. I just know that when I sit down to learn HARP it is much, much easier to do than RM.
*Not to say master, which is different. Chess is easy to learn, but very difficult to master.