I suppose the first question to really ask is, how many battletech players are into campaigns that last that long, with the equivalent being from Green to Legendary/Super Elite?
Is the typical battletech player mostly just someone that plays one-off games, or even a small string of games, or are they someone that is willing to play a campaign that lasts years? Sure, there are a few on this forum (I've run several long term campaigns). That is probably not the norm, imo.
The number one reason being lethality; in both D&D and Pathfinder, characters have lots of options to survive and/or be resurrected. While Edge can help, the fact is, battletech is FAR more lethal than these games. One good headcap ends the campaign for that player's character, whereas being reduced to negative hp in the other 2 games can be healed with any good healer class quite quickly. The games are designed for more heroic play, thus favouring not killing the characters, and with magic involved, that's easy enough to make it work.
An AC20 to the head, on the other hand, is a bit harder to justify survival. Sure, you can play with skin-of-teeth ejection rules, but even there, the character take... a LOT of damage if memory serves (4? 5? hits? without looking at TO). You can also house-rule things, I suppose, and hey, the Manei Domini had to come from somewhere!
In the end, there are already options otherwise; the narrative campaign out of Campaign Ops is basically a version of Adventure Paths. It can easily be changed to a format of a long-term campaign rather than a shorter. Each narrative campaign would be perhaps one long operation on a world (ie chapter of a Path) leading towards the grand conclusion as an example.
That being said, how I run a battletech campaign is very different from how I run my pathfinder or D&D campaigns. It has to be, based on not just the rules, but also the flavour of the setting itself.