Some collectors loathe the EU and look at it as "glorified fanfiction." So they won't buy anything, dimishing the market.
Some collectors embrace certain elements or stories from the EU and shun others, it is difficult to determine which ones people will like. I fall in this catagory, I love almost all EU but have minimal interest in the classic comics (I did grab a few of those comic packs though), the Legacy era or the KOTOR Comics. It becomes even trickier with which figures to produce. SOTE is a highly regarded EU source but people really have a dislike for Xizor. Only recently did my local TRU remove the six or so comic packs with him and cankles Leia from their pegs.
Some collectors embrace all EU and will buy pretty much anything.
I think the complexities of EU selection were just too much for too narrow a market, alas. Although, if memory serves, the demand for the EU wave in May was almost out of control.