Was reading interesting interview on Motociclismo (the browser translates it to English) and came across the interesting interview with a Piaggio Engineer. I always assumed they stopped making the big block due to emissions but apparently was more due to production capacity issues with the motor seemingly difficult and expensive to build:
motociclismo.it/moto-guzzi-vecchia-stelvio-figlia-di-organizzazione-molto-diversa-83979
motociclismo.it/moto-guzzi-vecchia-stelvio-figlia-di-organizzazione-molto-diversa-83979