IMHO it's not that Steam is that great, but rather than many of the major competitors have been quite bad.

Also, for a consumer there's a big convenience advantage to having a single store - from UX perspective (but probably not from the wider market leverage perspective, tragedy of commons yadda yadda) I'd very, very much prefer using a single service that's good enough to having to use three or more separate places that each individually are somehow better but each doesn't carry all the content that I want. The same applies to most other media - movies/shows, music, books.

IMHO the only way to get competitiveness would be to ensure that exclusivity deals are impossible/illegal, so that you might have multiple services each providing all content but competing on other qualities.