I've been asking myself the same question, and from what I've gathered so far, it seems that most of these artists consider their old goa work to be extremely cheesy. They probably
lost interest or evolved (like any artist), or just wanted to try something new and they liked it.
Maybe they are getting old and don't do drugs anymore like they probably used to, so there's less inspiration. Plus, let's face it, it's not a really big market
to try and make experimental dance music, so you just stick to the formula that works and hope for the best.
Also, their goa fanbase is far too small for them to care if they are 'true to the old school' when they can just make something that will certainly get them to the next festival/party.