Easy, specially if you compare his evolution as language designer in pursuit of minimalism, from his point of view both Xerox systems were too complex.
You can get hold of his opinion on this ACM session.
As nobody person in the world of computing, I don't agree with him though, I would rather have Active Oberon[0] than Oberon-07, his final language in pursuit of minimalism.
[0] - He did not took part in Active Oberon design and evolution, and it is closer to Modula-3 in features.
You can get hold of his opinion on this ACM session.
"Modula-2 and Oberon"
https://dl.acm.org/doi/10.1145/1238844.1238847
As nobody person in the world of computing, I don't agree with him though, I would rather have Active Oberon[0] than Oberon-07, his final language in pursuit of minimalism.
[0] - He did not took part in Active Oberon design and evolution, and it is closer to Modula-3 in features.