Why did the golden age of the Simpsons end?

I can't pinpoint it personally when exactly it ended, I enjoy some episodes into season 12, but I can tell you that two jokes on the show told me the writers have either gotten dumber or think we're dumber:

1) In some episode Smithers catches on fire, he rolls around on the ground saying "I'm flaming" and Mr Burns breaks the fourth wall and makes a goofy face like "he's gay, of course". Ugh.

2) Bart makes a reference to Bart Jr, making you think he's referring to his undercarriage only to find out it's a frog in his pocket. Not a great joke but what make it really, really terrible is the frog hopping out and ribbitting "I thought he was talking about his penis". Yeah, thanks for that pointless addition.

That kind of stuff, the utter need to explain jokes and treat viewers like they're morons. I don't tend to watch anything beyond season 12 because there's just so many landmines of explaining jokes and terrible writing like they just didn't care anymore.

As an aside: I get so many emails from Quora with vague questions like this just for answering a few Simpsons questions a long time ago. It's astonishing how many people can ask when/what was the best era of the Simpsons and when/why did things go wrong. Those questions must be on there hundreds of times.

Basically when the quality of writers declined and when everyone was going through the motions for money. It got old, stale and boring.