I'm guessing UA proved there wasn't enough appetite for alternative rules. Early UA tried a few expansions or alt systems (Greyhawk initiative being an example) but I don't think any hit that magic 70%. And when you're only doing 3-5 books per year, there isn't enough audience to support using a slot on OSR options, 4e options, etc. Even the most slam dunk of rules expansion (psionics) couldn't get enough support to pass muster. Ergo why most WotC expansion books have played it safe on new subclasses smaller system tweaks (downtime) rather than giving classes ADEU powers.