Americans didn't care for soccer as much back then as they do now
While interest in soccer is rising in North America, it hasn't reached the heights where it can compete with Euro soccer in terms of popularity and, to some extent, quality.
US club soccer can't compete with Euro soccer in terms of finances. The dollars just aren't there. There's a large segment of North American soccer fans who want US club soccer to be Euro club soccer. Same calendar, same structure with pro/rel, same finances. It won't work.
The Euro calendar won't work in North America. One need only look at MLS Cup a couple of years ago to realize that. If MLS, NASL and USL were one organization like the FA, you could begin to develop a system approaching Euro soccer with pro/rel. However, as long as they are competitors, it won't work.
Someone else has already pointed out that you still have cheap owners who won't do DPs. You still have teams folding in MLS; so the financial situation still isn't stable enough.
For now, as crappy as we all recognize MLS to be (in terms of rules and transparency), it's the best we can hope for now. Hopefully, those that care for North American soccer are watching the games and buying the products to generate the finances that helps US club soccer get better and better.