Personally, I'd say it was mainly because we were tech geeks who, back then, actually believed in a web that should be accessible to as many different platforms as possible using as many different browsers as possible, not just Windows and IE users.
In a sense, that notion prevailed: the web is ubiquitous, browser lock-in is considered a douche move and you can still access some really worthwhile sites even with rather simple means.
In another sense, we all failed, lending our hands to help create the current privacy-invading, dark-patterned UX nightmare we're still trying to pass off as a reasonable way of making the world a better place.
I never thought I'd lament the day Microsoft stopped making their own browser core. As a web developer, one less rendering engine to GAF about is nice. As a netizen, the Google dominance is going from unsettling to scary.