Pretty easy to understand why it's not more popular - while 5x7 contact prints can look very nice, they're smaller than most people want. 4x5 enlargers are common and nowadays often quite cheap (if you can find one locally or someone willing to ship, in which case the shipping is likely to be more than the cost, but still pretty reasonable.) 5x7 enlargers are far less common and usually more expensive. 8x10 is even harder to enlarger but big enough that many people are satisfied with contact prints.
Originally Posted by Bertil
5x7 is not more popular because it's perceived (rightly or wrongly) as being too big to easily enlarge while being too small to display contact prints. I toy from time to time with the idea of getting an 8x10 but not 5x7, for this very reason.