Regarding why the cruise lines go there--- there's only so many ports the ships can go and get back to from Fla. in a set number of days and a lot of those ports are already over crowded. One has to consider that there's many, many other ships and cruise lines cruising the same waters and ports and there is only so much room at these ports.
Progresso is a wonderful jumping off spot from which to visit one of the new wonders of the world---Chichen-Itza, a fabulous Mayan city. Uxmal is another Mayan marvel that can easily be accessed from Progresso. Merida,an old Mexican colonial city can easily be reached from Progresso.
Do a little research prior to going--view what's available out and about from the dock area and you just may be surprised.
If one's looking for another St. Thomas shopping mall atmosphere, then you could expect to be disappointed in Progresso but if one's looking for something different then it's worth considering.