Who needs Instagram when you can put cheesy filters over your crappy phone photos from right within Twitter itself?
Twitter first began offering photo filters in 2012, and it's continued its efforts to make Instagram irrelevant ever since.
There is an argument to be made for simply posting to Twitter instead of going through Instagram.
Your accounts are probably linked anyway, so your Instagram photos get tweeted automatically. And this way your photos will actually appear natively in your tweets, instead of in a link your followers have to click.
But most users who care about this are probably already invested enough in Instagram that they don't want to quit, barring some moral objection to Instagram owner Facebook - which, to be fair, isn't that far-fetched.