Dilwale Dulhania Le Jayenge or DDLJ is the most successful Bollywood love story ever made. It gives us the Sharukh/Kajol Duo. Even after it they both acted as couple due to audience demand followed by Kuch Kuch hota hai, Kabhi Khushi Kabhi Gum etc.
At the time it was released, it made too many new records and broke old ones. This made it unforgettable.
Why it broke record because people loved it, as vj1207 said, Maratha Mandir is doing good business and it also shows other films too. DDLJ only hold one show slot and is still showing.
It also got support from Yash Raj Camp too.
In early 2011, a theatre strike threatened the film's uninterrupted showing streak. Producer Yash Chopra contacted theatre owners to try and ensure that the film would continue. He hoped that the film would continue to run for at least 1000 weeks. - Wikipedia
It's also famous for Jokes too but it's commonly used scene in recent flicks and TV shows too as homage.