Thinking about this in particular is interesting to me.
For games like NFS, weather simulation and its impact on the road is basically "it rains, road gets slippy", where as sims want to go further and model how track camber and bumps impact water run off, which is shown how GT7 tracks have clear paths that form as the track dries up, as cars pass over and the water disperses.
For the tracks that we do have dynamic weather available, could we assume that those are the tracks that PD checked thoroughly that everything is simulated accurately, and the other tracks they just didn't have the time to, or have decided it just isn't important enough to implement and check?
I agree that weather should be available on all tracks, especially if they're pulling regional meteorological data, as other people have pointed out some of the American tracks should definitely get bad weather once in awhile. So the reason they don't, to my mind, has to be a query of did PD decide the time required to implement and tesñt just wasn't worth it? Did they think the handful of tracks is enough? Maybe they thought not enough people would want chllenging weather on every track?