Most cathedrals and monuments are like that because until recently in human history, they took a long time to build and so the original architect would die, the financing might collapse, etc. Heck, this happened to Gaudi; the remarkable thing here is that the people after Gaudi wanted to continue his vision as much as they could.
The Washington Monument in DC, for example, famously is different colors because they had to change the source of marble during construction when funding halted for a time.
If would likely be even frillier, more colourful, and more organic if Gaudi had seen it all the way through.
It's still one of the modern wonders of the world.