I couldn’t find information about the history of subtitling, so here is a thought about what might have happened.
Since TV is mass entertainment and mass entertainment is sensitive towards audience displeasure, people complaining about weird accents might have been the reason why we now see that habit of adding subtitles to people speaking in a familiar language but with an accent. Often the accent doesn’t really make it more difficult to understand, yet there are subtitles anyway, which can seem insulting.
And since I have studied people’s behavior a lot, what I could imagine is that this was caused by people who simply don’t WANT to understand. There might have been racists or just general pricks who went like: “What’s that Asian guy mumbling about there? I can’t understand a word of that gibberish!” while they could have understood perfectly well if they hadn’t been an ass. And then the entertainment business got afraid that this might harm the audience appeal of the product, so they better added subtitles, just to be on the safe side.
And as is so often the case when people act out of fear, they do a disservice, and to themselves, too.
Again, such things are often merely about willingness to make an effort, even if just a small one, and many people are not willing. It’s the same issue with people speaking foreign words the wrong way just because they want to signal disrespect; are not willing and don’t care enough to make a little effort.
When people admire that I am good at speaking certain foreign words, it gives me mixed feelings, because all they’re really admiring is that I make a little effort and they don’t even try. Many things are damn easy if only you can get yourself to have a bit of interest. And thus, interest is what paves the way to excellence.
If you think “Nobody likes to have a speech impediment.”, you’re wrong. Many of them are even proud of their self-made one; created with the power of the mind.
See this video for a humorous (yet with a deeply serious core) look on the issue:
This doesn’t even cover the more extreme degrees of the issue, like phonetic mistranslations. One example of that that made me facepalm for years is cherries named Schattenmorellen in Germany. Schatten? Did they grow in the shade? And what the hell are morells? Nope, they were originally called “Chateau de Morel” (after Castle Morel), but someone decided that making themselves look like an idiot/jerk by doing this crude phonetic translation was a good idea.
Beef jerky is another example of an equally dumb phonetic translation. That product was apparently named by and after knee-jerks.
UPDATE 2018-12-26: The Wall Street Journal managed to go far beyond this in what one has to assume is politically motivated racism, and the contained cynical self-parody (subtitled Chinese perp vs. non-subtitled Israeli victim) could also be interpreted as antisemitic, ha-ha: