I would say it is because of Gary's personality. He's a blue collar coach and he wants players that are going to buy into his system and play with discipline, and he wants players that will stay with him for 4 years. He usually stays away from big name high school All Americans because they are high maintenance and they come into college thinking they are bigger than the team. And don't forget that Gary won the National championship in 2002 with a team that did not have a single McDonalds All American on the roster. As a matter of fact, his first recruiting class after the national championship season had 2 McDonald's All Americans, Travis Garrison and Mike Jones. Both of them were underachievers at Maryland and you could tell that Gary felt like he sold out by recruiting them (Mike Jones was the #2 rated player in the country in high school - who was #1 that year? Lebron James

). He has not recruited an All American since.
Gary built his reputation by recruiting players that other top teams would not consider "elite"(think Lonny Baxter, Juan Dixon), so they always come in with a chip on their shoulder which fits Gary's personality perfectly. And he is well regarded as a coach that plays by the rules in recruiting, and a lot of times you cannot recruit All Americans unless you provide some "extra incentives".
I will point you to the case of Rudy Gay, who is from Maryland but went to UCONN: UCONN's coach
Jim Calhoun paid Rudy Gay's AAU club team 25,000$ when Gay was in high school. Amazingly, this was not against the rules, but it is the kind of garbage that some coaches do to attract top players. Gary never does those things.