JPEGs are easier on storage than equivalent PNGs when dealing with photos and photorealistic content. As that's mostly the sort of content Twitter gets they've probably decided on JPEG only simply to keep costs down. You don't need as many drives to store 5 billion JPEGs as you do 5 billion otherwise identical PNGs (unless they're mostly non-photo-realistic images).
That's the only real reason I can think of that makes sense. Doing it as a subversive way to get people to stop posting art won't work as they'll just decide to live with the JPEG artifacting or go elsewhere. It should be noted that they're not only making changes that affect PNG files, they're also going to re-encode JPEGs at 85% JFIF quality if their uploader software deems the original JPEG to be too high-quality.
Still sucks though. JPEG and PNG both have their strong and weak points so forcing everything to be JPEG hurts cartoonists. Maybe this will lead to a resurgence in artist websites as it'll be harder to post high quality images to a public service? Back in the days before social media and DA artists usually made their own websites to host their content because there simply wasn't anywhere else for it to go.