Ever since ESO released on there have been - as I can hear from thousands of reviews on Steam - several issues with downloading and playing the game through that particular platform. Out of 17,400 reviews, about 20% are negative and a vast majority of the negative reviews are complaints about these particular issues (go to
http://store.steampowered.com/app/306130/, then filter only negative reviews).
I've been playing ever since the release and thus don't own a Steam copy of the game, but, honestly, if there are so many customers that can't play your game, the issue should've been addressed by now and most definetly fixed. It's been, what, two years since the game released on Steam?
That being said, I've no idea whether ZOS has actually addressed the issue. If they did, please post a link. I just want to know more about it since I'm seriously questioning whether ZOS is ignorant or there are so many people on Steam who are so stupid they can't handle a simple client.