I Am is underrated because the expectations were set too high, and there was no way Nas could live up to them (on the other hand, Stillmatic is a bit overrated because most people had buried Nas and weren't really expecting much)
Why were the expectations and anticipation so high?
1. It was Nas's first new album in almost three years
2. It was supposed to be a double album
3. It was his first album since going from Nasty Nas to Nas Escobar, and everyone wanted to see if he'd bring back his "Nasty Nas" style. After the backlash against It Was Written (and The Firm), people expected Nas to realize his missteps and take it back to the style purists loved.
4. "Nas Is Like" was incredible, and basically the perfect lead-in single for people expecting another Illmatic. It was so good, it really made people believe he was taking it back to Illmatic. (When the album arrived and nothing else came close to "Nas Is Like", it was anticlimactic)
5. The emergence of Jay-Z (and, to some extent, DMX) as huge stars made people eager to see if Nas could take back the spotlight
I can say personally that I probably never anticipated an album as much as I Am. When I first heard "Nas Is Like" on a January night while listening to 92.3 The Beat, it made me anticipate it THAT MUCH MORE.