I don't like the new industrial mechs, what are they for, cannon fodder?
Industry. ;) Also, they can turn up in roleplaying campaigns and guerrilla conflicts.
And why change the partial cover rule?
Player feedback. The old rule was unpopular because partial cover - which is supposed to cover the target and make
it harder to hit - suddenly turned the head from a 1-in-36 target to a 1-in-6 target. Partial cover was a godsend for decapitation instead of protection. The playtested fix was to keep the standard to-hit chart, where the head remains a 1-in-36 target, and discard leg hits. Because go figure: you're hiding 'Mechs behind hills. If you got weapons that can shoot through hills, then you've got 'Mechs with WarShip weapons.
Regarding your subject line, there's been very little drift in the rules since the 2nd edition rules set. 'Mechs made in 1985 are still playable in 2018. Yes, rules have been tweaked here and there but compared to, say, DnD or Shadowrun, there hasn't been much change. BattleTech's change is driven by player feedback, questions on these forums that highlight errors or problems with playability. Listening to the players has been very helpful.
Also why so much fluff in the newer books?
A lot of the old fluff was out of print. The new corebooks were a good place to republish the information and get some new stuff in. Shadowrun had been demonstrating the value of richly illustrated, fluff-rich books, so BattleTech got the same.
I just want the rules down and dirty.
Look into the boxed set, then. That strips out most of the fluff. There are also quick start rules available