Most of all: it was new, it was fresh at that time. There wasn't such a film since the old cliffhanger shows from the 30s. The mere idea of making a lovable hero stumble from one exotic action sequence to the next doesn't warrant a great film yet, but if you have the talent to make the acting, locations, effects and even the dialogs so much fun, convincing and effective that two hours pass like 10 minutes, you get a classic and that's exactly what happened here.
I don't even have to go for the nostalgia effect all that much, I didn't get to see the film before the early 90s. Okay wait, even that's already 20 years ago

The Mummy is a fun film too, but it simply wouldn't exist if it wasn't for Mister Jones.