Because they're the most fundamental aspects of positional chess and the attack. You target the opponent's weak pawns and try avoiding weaknesses without compensation (piece activity compensates weaknesses). They are accessible weaknesses usually.
They are weak because other pawns cannot defend them and generally if you have a weak pawn trade heavy pieces and keep minors since the heavies can line up on the isolated pawn and force you on the defense of it leaving you passive.
I have been doing some reading and studying since I have gotten into chess, and i was wondering why so much effort is put into attacking and defending isolated and backward pawns.