This answer will add to/repeat some of what's above.
Although Roger Ebert's fame has been more enduring, they were "Siskel and Ebert" at the time. They were talented, smart critics, but the key was accessible. In the 1970s and after, major film critics could be problematic, in evoking New York City and/or academia (Kael, Sarris, Kauffmann). At the other end, Rex Reed and Gene Shalit were TV caricatures of the critic. Siskel and Ebert were in the middle, also literally, being from Chicago.
The other key fact, of course: the Internet didn't exist for most of the S&E career. It's impossible for critic(s) to have the same impact, now. At the time, being a film critic was vaguely ridiculous to the average person; S&E broke that resistance, legitimizing the profession. Both were genuine newspapermen, but if Siskel was still vaguely elitist (in fact, a Yale graduate), it was easier to imagine Ebert bending elbows with Jimmy Breslin, et al. (Edit: I may have overstated here. They were both popular, but Ebert was closer to an everyman.)
Two other factors: self-promotion and longevity. Ebert had the necessary knack of occasionally stirring controversy, as with his oft-mentioned review of Night of the Living Dead, which decried parents taking kids to see the film. He also panned Blue Velvet. Right or wrong, he got attention.
Longevity: They met a need for televised film criticism, a half-hour per week, but Ebert outlived Siskel by 14 years. "Siskel and Ebert" ultimately became "Ebert and Roeper," but after Ebert left TV, the show faltered, leaving his name the common factor. Even so, he remained prolific, continuing to work despite major health issues. This adds to his legend; in this sense, he bears comparison to Christopher Reeve.
The bottom line: Roger Ebert was a great (Pulitzer-winning) film critic. If he wasn't better than the other greats, he was in the right place at the right time, and made the most of it.