Hi,
If you look at any forum/site, you will see that people are more interested on Yaoi and rarely on Yuri. The quantity of manga for Yaoi manga is not the same as Yuri as well.

My theory:

Of the mangas that have sex in them:

Generally, Yaoi is written for a female audience.

And hentai is written for a male audience, and there is A LOT of hentai.

Yuri is mostly written for males (and many times mixed with hentai), though there are several really good deep yuri that's not just about sex that doesn't have a specific audience.

So, for the most part, males have a lot of hentai written for them, whereas women mostly have yaoi. Therefore, the audience for yuri is a lot smaller, and it's not as popular.

This question has been asked loads of times. You get the impression that it's because yaoi appeals to a avid female fanbase. Yuri is believed to be more popular with males, but that doesn't seem to be the case. In the recent MangaGamer Survey they found that while BL and otome was overwhelmingly popular with women and SoL and nukige (eroge) was popular with men (with romance demanded by both genders and all ages), it was surprising to find yuri was liked equally by all groups as well. Although not as popular, yuri isn't dismissed as a boy's interest. Both genders and all ages tend to like yuri, while yaoi tends to be polarized.