They've been buying Marvel for a lot of reasons other than price.
Just because DC went to an even lower cover price doesn't mean Marvel readers would automatically jump to DC because of price. (Hell, if a $2.99 DC cover-price vs. $3.99 Marvel cover-price didn't already cause people to drop Marvel completely, who's to say a $1.99 cover price would cause a significant change in people's preferences/buying habits?)