Eddie Guerrero and Dean Malenko did get a chance to work against other opponents. Dean Malenko held the television title, feuded with Taz, and even had a short run against the Tasmaniaz. Eddie Guerrero doubtless would have had other feuds had he stayed with the company.
Chris Jericho was very much unproven in the U.S., outside of his work in Smokey Mountain Wrestling. Chris Benoit left before he had much of a chance to shine, and again, he was not anywhere near the name he is today. In my view, ECW did use them as well as they should have at the time. They put them in the ring with other great wrestlers, and had them put on good exhibitions of wrestling to present a break from the mindless violence.
Guys like the Sandman and Tommy Dreamer were pushed because the great wrestlers never stuck around, instead going for the big Two, which they should have. The guys that ECW were left with were the cast-offs, who had obvious flaws. ECW patterned their style of wrestling around those guys, so they could hide their flaws as long as possible. At times, ECW created memorable characters, and that kept them popular for a long time.