Well, as the resident LEO. I can answer that question for you.
Laziness....
I'm a officer in Miami Florida. Our weather is far hotter than what happens in LA. I wear my vest everyday I'm on duty. It's hot, kevlar is still hot. Sure, it's not like the stuff from twenty years ago but it still gets hot under the vest. Right now the weather is in the 70Fs and I still sweat under the vest.
Most of the veterans on the force (5 years or more) don't wear their vests because they fall under what I call the superman stage of law enforcement.
Stage 1 - First Five Years
Trust no one and wear every single piece of equipment
Stage 2 - Second Five Years
Think you're bullet proof
Stage 3 - Third Five Years
Have a close call and wear the vest again
Stage 4 - Last Five Years
Think about Retirement
I believe that I'll never leave Stage 1 for one reason. I've already been wounded in combat. I wear my vest for everything including off duty details.