Academics usually like shiny things, specially free. And Mendeley didn't target the old school ones, they targeted students, post-docs, usually the trend-setters in the academic environment. Professors and Senior Researchers are too busy or don't have time to check these things, and they end up using what other people use in their labs.

And for broke, internet saavy students, free stuff with a promise of a new groundbreaking trend in science was enough to lure them.