it's quite simple, actually. i guess i am confused about what you are confused about. there are way LESS nonstop flights from NYC (LGA AND JFK/EWR, not just LGA) to south florida than there used to be.

although i mainly fly CO, i do check DL's prices and i have noticed a reduction in service from NYC (taken collectively and not focused on LGA) to south florida (taken collectively and not focused on FLL).

point of the OP, if i may:

total # of non-stop flights from LGA-JFK-EWR to FLL-MIA-PBI one year ago < total # of non-stop flights from LGA-JFK-EWR to FLL-MIA-PBI today.

that is undeniably true. it's simple math.