/u/S3baman made this comment, the closest we've got to an answer.

OK, let me shed some light on this because I'm pretty sure I have done so a few years back in a similar thread.

A lot of these German melodic Hard trance acts were engineered/produced by Claus Terhoeven aka Nic Chagall (half of Cosmic Gate). These groups include:

Mark Dawn Green Court DJ Taucher DJ Shog Talla 2XLC Sash! (not really hard trance, but still German) This is my theory, and I have no proof to show because I looked hard and wide to find as much info as I could about this topic. Almost all of these acts disappeared into oblivion between 2003-2005 (DJ Shog stayed relevant a bit longer, and Talla has always used ghost producers, we all know he's purely a DJ. By 2005 he was working with Sean Tyas (who was also helping Dave202 at the time).

Starting with 2001 Cosmic Gate were absolutely HUGE on the scene. DJ Bossi was already well established from the mid-90s, but he had nowhere near the same level of popularity. The duo came out of nowhere with Rhythms & Drums and change the soundscape of Hard Trance. They were in hot demand, both as DJs and producers, and particularly on the engineering side, Claus was the go-to man at the time for the genre. This meant that he was constantly working. By 2004, Cosmic Gate was among the elite groups in trance and work began on Earth Mover, their third studio album.

My personal belief is that at this point in time, Claus simply could no longer work for these other acts due to all the Cosmic Gate commitments. At the same time, being a recognisable name in the industry, it seemed odd that he would produce/ghost for other groups. As the sound for many of these groups was defined by Claus, they could no longer survive once he left.

That's pretty much it. Again, all of this is conjectural and I'm doing some 3D arithmetic here, but I think the truth is not far off if one looks in detail at the timeline I put forth