Mamma Mia is just bad with really bad songs which do not relate to the story. Awww...how cute. Except that it's not, it doesn't and they do. Mamma Mia is effervescent ebullient fun, which does not translate to "bad". The songs have been legendary international hits for over 30 years. And most of them have successfully found their way into the book directly relating to the plot.
Because it's mostly for people who are too tourist to know better. Then how do you explain the millions of locals in every country it plays enjoying the show as well?
I always think it's funny how the musical theatre "fans" (snobs) come out and pretend that any show written and designed as popular entertainment and overwhelmingly successful is automatically dismissed and considered somehow unworthy of Broadway. I guess it is supposed to look very intellectual, impressive and way kewl. But if you actually look to Broadway's roots, you will see that the likes of Mamma Mia are actually closer to the original spirit of Broadway than most of the faux-elitist picks. There is plenty of room for both.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian