Now I have not played BT in ages, but from my time there part of the stability I saw with BT is that its a bit more straight forward in many ways.
Sorry I'm going to have to disagree with you there pretty strongly.
First, Battletech has multiple layers of rules that have to remain compatible (at least in theory) with each other. There are the standard tournament rules, construction rules, advanced optional rules that cover areas in the previous rule sets plus campaign rule sets at both the tactical and strategic levels. The exceptions being Alpha Strike and A Time of War the former being a "fast playing" version of the tactical game and the latter being the Battletech RPG game. They are based on the core game but they don't have to remain tightly compatible.
Second, while Battletech proper does only have approximately two skills it does maintain literally hundreds of cannon units most with multiple variants. All of which have to be checked for legality anytime a rule change is proposed. Producing new sets of units of any stripe that are verifiable of legal construction is a very time consuming process. Also, as a design decision made long ago, Battletech does not have the luxury of completely reinventing itself every ten years or so. Imagine if Shadowrun had to maintain its rules so that characters made in 1st edition could be played in 5th edition with no conversion? Granted it's not really an apples to apples comparison but that doesn't negate the complexity of trying maintain rules without invalidating units built almost 30 years ago.
I will grant that Shadowrun has multiple "battlefields" if you will though I will challenge the fact that makes the Battletech "battlefield" less complicated or easier to balance. When you start taking into account Battlemechs, vehicles, infantry, fighters, artillery and (God help me) warships has been an ongoing process for as long as I can remember. And don't even get me started on Inner Sphere tech vs. Clan tech, ack!
I knew you would bring up the fluff thing and I will admit from a personal standpoint that when I started playing back in the 80's I didn't give a lick about Great Houses or ComStar or any of that crap. But the fact is fluff is just as important to a lot of Battletech players as it is to any Shadowrun player. There is also a whole lot more of it to keep track of. Imagine if Shadowrun were currently in the year 2275 and they had more than 250 years of history to keep track of rather than the 70 or so they do now. Let's also not forget the number of players out there who would just plain freak if you so much as suggested that they use a unit that doesn't belong to the chosen faction. Which is yet another layer of complexity managed by volunteers. All those hundreds of units and someone has to go through all that material and figure out what faction has access to what and during what eras spanning hundreds of years.
So while I would be the first to admit that there are different challenges to design, edit and maintain between Shadowrun and Battletech I wholeheartedly disagree that either is easier than the other.