That’s a really interesting question, since it hinges on what exactly it is about observances that appeals to the “semi-faithful”, the people who are normally lukewarm about such things.
I think one thing you have to note is that Ash Wednesday is the most visible of the Catholic holidays, where the individual observant is concerned. When you go about your day on Palm Sunday, or All Saint’s Day, nobody knows by looking at you whether you went to mass. On Ash Wednesday though, especially in largely Catholic communities, everyone can see who are the lazy ones who didn’t bother to get down to church. So, I think simple shame is a part of why the lukewarm folks get down there on that particular day.
Many non-Catholics come to Mass on Ash Wednesday to have ashes put on their foreheads.
All in all, I think Ash Wednesday is a favorite among all Christians.