Well, 'cause Kishimoto hasn't showcased him. Hell, remember Chouji? Who the hell thought he would pull that buttefly mode out of his ass? Anyone's capable of anything in the series. Hell, Konohamaru pulled a fast one on Pain. Who'd've thought? But, going by what the manga has stated about Suigetsu, even if he hasn't fought other ninja per se, I could argue the same against Neji. In fact, allow me to be the devil's advocate against Neji. He lost to
Naruto. Naruto
without the Rasengan. He then almost died to Kidoumaru. The only reason he killed Kidoumaru was because he didn't cut the web of the arrow after it pierced Neji. In the end, Neji was running away from him and achieved his goal because he was lucky enough to get the opening to send chakra back to Kidoumaru. That match was a draw. He's alive thanks to Tsunade and Shizune. So basically, up to that point, Neji's got 1 single win against a still unskilled Hinata, which was a totally unfair matchup. Afterwards, he only fights again but against himself, and he had a shitload of trouble but managed to defeat the clone.
Therefor, following logic, I'd say Neji is overhyped if we see his match record vs. what's actually
said of him. Those who think so highly of him is because of what they believe he's
capable of and what's been commented of him in the series. The same could then be said of Suigetsu. We haven't seen enough action per se, but he has been talked about and hyped to a certain degree. He was chosen by Orochimaru and then handpicked by Sasuke for Taka. That says something.
Oh, and don't get me wrong. I'm a big Neji fan. He should seriously get some showcase in the manga (though I like the comic relief he's dishing out

). I mean, he got outshined by Chouji.
Chouji.