Because so many makers had used the S, T and P shapes for so many years (without a peep from Fender) that the shapes ceased to be a unique signifier of who made the guitar. The LP shape hasn't been quite as widely copied, so Gibson was able to register the shape as a trademark. But as has already been posted, they weren't able to prove likelihood of consumer confusion in the PRS case. Gibson has the trademark on the headstock, not on the Les Paul body which is one of many reasons PRS won the case. Fender only has the headstocks protected. What sunk them was in the 80's when they had gone to other builders telling them they couldn't use the Fender headstock shapes. When asked about the body shapes Fender said those were okay they were only concerned with the headstock. IIRC Tom Anderson is the one that had his papers from the Fender legal team stating such and presented them to the court.