It's pretty basic, Wonder Woman while being a great female icon and recognizable to the general public just doesn't sell comics. If you look at the sales numbers in the past 20 years.....
2007 to 2010 - The last reboot with Allan Heinberg, who coming off a popular marvel run and being a tv writer couldn't even finish a single year of issues. First few issues sold well, over 100k but by the time he left at issue 6 it was down to 50k area and even after adding a big name writer in Jodi Picoult it still continued to spiral. Once Gail came aboard the numbers sorta solidified but by that time it hovered around mid to high 20k.
1987 to 2007 - Perez rebooted the run in 1987 with a lot of fanfaire and may be the most well looked upon writer/artist for WW. Sales numbers are hard to come by back then, at least good numbers but it seems with Perez in the beginning the numbers were 80K plus for a while, with the first bunch over 100k. Over the years after Perez, you had Messner Loebs, Byrne, Simonson and then Rucka, who finished up Vol2. For the most part after Perez left the numbers through out averaged about 35k to 45k, with big issues like 100 and 200 getting double that. When Rucka closed down shop, the last issue of Vol2 sold about 45k.
So if you take the last 25 years, her single solo book sold about an average of 40k, not really good numbers
She has always had just one book while Superman and Batman for the most part had solo titles, group titles, family titles, etc... DC has always kinds treated her as small part of the so called trinity. I honestly couldn't even tell most folks who her fans are within the comic community. My LCS orders about 6 WW issues a month, with 2 of those going to reserves. I called around to shops locally to try and get the AH! variant cover and only 1 out of the 6 shops I called was actually ordering 25 to get the cover, so she is almost at an all time low in popularity among those who actually purchase comics. So while she is popular in media to some degree, the people who actually spend money on her are pretty limited, so DC had to try something.
I have every issue starting with Vol2 and about 100 odd issues prior to that reboot, I've been a big fan of hers for a long time and even I knew something had to give.
Evan