The weather and the beaches - those Brits and Scandies and Germans need somewhere to go. Majorca, Ibiza, Corfu, Mykonos... these are all major tourist destinations. I last visited Barcelona in the late 1980s, so I don't know how it has changed. I assume Las Ramblas is still the area to party.
Las Ramblas is still the party destination. Sort of. Its a bit tourist honkey tonk. What it actually is is a broad boulevard with a wide pedestrian promenade down the middle.
There are actually several “ramblas” very similarly laid out, but none of the others are really destinations.