That's another thing about online streaming and downloads, they tend not to have the features or commentaries, another reason I'm reluctant to replace DVDs & Blu-Rays with them.
The audio commentaries for early Simpsons at least are available online, people tend to rip and dump them on various outlets like Tumblr and what not.
The only thing about audio commentaries and special features in my own personal opinion (this is not limited just to The Simpsons but other TV shows and films that feature them) is that the vast majority of them are only beneficial to hear or see once and they tend to be of cheap filler quality designed to fill space up on the disk and look impressive on the packaging.
There are internet sources for fan audio commentaries for films and TV shows as well and even reviews of official audio commentaries - ratethatcommentary.com (site is nothing to do with me, just mentioned for purposes of discussion).
Larry the Loafer gave kudos