Comment by notatoad

video decoding on a general-purpose cpu is difficult, so most devices that can play video include some sort of hardware video decoding chip. if you want your video to play well, you need to deliver it in a format that can be decoded by that chip, on all the devices that you want to serve.

so it takes a long time to transition to a new codec - new devices need to ship with support for your new codec, and then you have to wait until old devices get lifecycled out before you can fully drop support for old codecs.