That might perhaps be because the total was not very impressive? 11 to be precise. Six less than Agassi, that had to hurt.
LOL Sampras couldn't barely give a shit about the masters series event and you could see that with his attitude and the way he approached many of them during his career. He said himself many times that 'no-one cares how many times you win events like Cincy', and without looking it up I bet many people don't know that he won that event 3 times. It can't hurt him at all when he never cared about his masters series title count in the first place during his career, and after he retired he forgot about many of those events he won.
I wouldn't be surprised if Agassi doesn't even know that he won 6 more masters series titles than Sampras. A lot of fans don't realise how little players actually care about their career stats/achievements after they have retired and even during their careers in many cases, especially compared to those of us on tennis message boards. Sampras for example has talked about beating Becker to win his first Australian Open title in 1996 (I count 3 mistakes there), beating Agassi to win Wimbledon in 2000 and skipping RG in 2002. This shows how little he remembers or cares about his career activity.
Also current generation seem more squeaky clean compared to previous ones, when players like Sampras, Agassi, Borg, McEnroe and even the great fighter Connors regularly tanked matches at non-slam or less important tournaments when they couldn't be bothered.