It's because in the 90s - mid 2000s it was made to be for all ages, from small children to teenagers to adults. Then when Stephen Hillenburg left they changed the style to mostly just for kids, so anyone over the age of 11 or 12 doesn't really find it funny anymore.

Because the spongebob hype has ran it's course people just got sick of that little yellow sponge

The new episodes of Spongebob are just gross now.

they probably ran out of ideas for episodes or it gets boring seeing a yellow sponge all the time

sometimes living in a pineapple under the sea isn't all its cracked up to be

It never was funny.

Cause you stopped smoking pot.