I think this is because creating a Javascript framework is easier than creating a framework in other languages - clientside or otherwise.
Note: I didn't say making a good framework was easy, just making one at all.
Sometimes people just decide to experiment, to try something and then as they grow with it they think it's big enough to share. When a whole bunch of people do that, you end up where we are with JS frameworks.
I'm in the "it's no big deal" group. Are there thousands of frameworks? Yes. Have I heard of more than 5 of them being used extensively? No. The good ones are embraced by developers and shared. The other ones are mocked into oblivion. It's the natural order of things.
Note: I didn't say making a good framework was easy, just making one at all.
Sometimes people just decide to experiment, to try something and then as they grow with it they think it's big enough to share. When a whole bunch of people do that, you end up where we are with JS frameworks.
I'm in the "it's no big deal" group. Are there thousands of frameworks? Yes. Have I heard of more than 5 of them being used extensively? No. The good ones are embraced by developers and shared. The other ones are mocked into oblivion. It's the natural order of things.