Any answer to this question is going to be uncomfortably close to opinion rather than fact, but I will have a go at answering it objectively.
Travolta and Willis were - by far - the most well known actors in the movie. Travolta in particular had been around in leading roles since the 70s with major parts in Grease, Saturday Night Fever. However his more recent successes were very light material such as Look Who's Talking. Willis had not enjoyed much great success recently either but Die Hard was not that far in the past compared to Travolta's heyday in the late seventies.
The other actors in the movie, whilst known were not big stars. Roles in Dangerous Liasons And Baron Munchausen were probably Thurman's biggest to date. Likewise Samual L. Jackson has appeared in a lot of movies but in relatively minor roles.
Then there was the the story involving Mia (Thurman) and Vincent (Travolta) in the movie. We get to see Travolta at his very best - good acting, a funny and dramatic plot, and he gets to remind us that he can dance a little. Coupled with the more comedic moments with Jules book ending the movie, His character is arguably the most prominent in a very well regarded movie. As Nobby also points out, the role of Vincent Vega as drug-taking hitman is vastly different from the roles he normally takes, particularly the role of James in the Look Who's Talking movies.
So Travolta's position as a fading star coupled with him being attached to such a prominant role in such a well regarded movie is the reason why this is remembered as the movie that re-launched his career. It did no harm to Jackson and Willis' careers too, and Thurmann goes on to do Kill Bill with Tarrantino, probably the role that she will be remembered for.
The movie had a lot of buzz about it at the time. It was only 2 years after Tarrantino has emerged into the mainstream consciousness with Reservoir Dogs. Reservoir Dogs was a very controversial movie at the time, and the follow up garnered a lot of free attention as a result. I am sure it got good publicity, but it was a bit of a perfect storm that Tarrantino is unlikely to be able to repeat.