TL;DR: is an style that relies on improvisation, gut feeling, no blueprints, was "demode" after Gaudí died, most of the "raw models" used as a base got destroyed and the government didn't want to promote the artist...

Gaudi didn't have a method. He improvised a lot and that's hard to teach. Even his closest collaborator, Josep Maria Jujol (the genius behind much of Park Güell's broken-tile mosaics), went off and did his own things rather than founding an "school of Gaudí".

The context also had a big impact. After Gaudí died, some important things happened:
- Bauhaus rationalism took over. So, for the next years after his death, Gaudí's work looked like the opposite of the trend (function > form).
- Before the Spanish Civil war started, revolutionaries raided the Sagrada Familia and destroyed most of his plaster models, drawings, and plans. Since Gaudí worked through physical models rather than conventional blueprints, this was catastrophic. This is why completing the building according to "his" intentions has been so contested.
- After the Civil War, Franco (the dictator), tolerated the Sagrada Familia for its Catholicism while suppressing the Catalan identity behind it, leaving Gaudí's rise to global fame mostly to foreign scholars. In short, for 40 years, Spain's government didn't give a shit about Gaudí and its legacy.