Is this due to the character builder or the change in license between 3.5 materials and 4e? I am inclined to think the license change had more to do with it than the character builder.
I would be inclined to think it was a combination of the two, and the turn of events which happened.
The draconian first version of the 4E GSL wasn't particularly appealing to many 3PP companies, which most likely put many of them out of the running. By the time the less draconian second version of the 4E GSL was released, it was probably too little too late anyways. The companies which initially opted out, already had other plans without 4E D&D. (ie. Paizo, Green Ronin, etc ...)
So initially, the 4E GSL license probably prevented a huge flood of 4E 3PP supplement books being released back in mid-late 2008 to early 2009. At the time, I don't remember the DDI character builder having a huge hegemony at first. In my 4E games and others I was aware of, I noticed the players were starting to really use the character builder a lot more sometime in early 2009. By the time it was Gencon 2009, it was clear to me that the DDI character builder was very popular and was the "hegemon". Even in my own 4E games back in early-mid 2009, I allowed the players to use any 4E 3PP books for their player characters. But none of them took up my offer, and preferred to use the DDI character builder. Other 4E DMs I knew of locally, mentioned the same thing in their groups.
Fast forward to today, there doesn't seem to be much of a market left for non-WotC player's option 4E crunch. I suspect even if WotC suddenly today made the 4E license into something as liberal as the OGL, I doubt there will be many more 3PP companies signing up to crank out more 4E 3PP supplement books.
Going back a decade to 3E D&D, a lot of the initial 3E 3PP stuff was DM stuff like modules and monster type supplements. (I imagine this is what Ryan Dancey et al were originally thinking of, when it came to making the 3E license relatively liberal and the types of books they wanted 3PP companies to produce).
The player's options crunch heavy stuff didn't seem to take off until sometime in late-2001/early-2002, with titles like Malhavoc's "Book of Eldritch Might", Mongoose's "Quintessential Fighter", etc ...
Mongoose Publishing : For All Your Gaming Needs ...
The conjecture is whether titles like "Book of Eldritch Might" or "Quintessential Fighter" would have sold like hotcakes if there was a fully functional 3E D&D character builder done back in y2k. (ie. Not the bug ridden one from back then).