Well, Total Warfare is not in print at the moment, right? Between MWO, BattleTech video game, new intro boxes coming within a year or so, this is as good place as any to release a compilation rulebook. Even so, BMM, is not a complete collection. It includes all major 'Mech weapons but not everything, so waiting till 3250-era wouldn't still make it "everything in one place" book.
Also, though IlClan is coming, the next era is likely still way off.
The latest info was "2017 release for IlClan unlikely". Let's say first quarter next year, optimistically. Then new box sets, and maybe Combat Manual Davion. That would be three major products for the next year. I would bet any 3250-era stuff is 2019, earliest.
Second, we don't actually know if there is any new tech in IlClan-era. For all we know, tech could be "redistributed" or something (Clan tech restored to top dog place, with no common mixed tech), with some items made extinct. For all we know, IlClan/3250-era might introduce new rulebooks, new ruleset. For all we know, it will be just the usual tech addition.
So, no, timings aren't really odd.
EDIT I have vague recollection of someone saying IlClan (the book) won't include new tech, all Dark Age tech exists right now. But don't quote me on this, and anyway even if i was right, plans could've changed. If it does include new tech, it is likely experimental stuff, not BMM stuff.