There’s multiple possible reasons.
- RCI need real path finding connections between them
- RCI only commute for “1 hour” and won’t accept any target after that. traffic jams increase travel time. taffic balances out parallel roads, which helps to avoid jams and increase travel distance. rail is much more expensive but got endless capacity (no traffic jam) and is fast and for that reason allows high travel distance.
- Maxed out R/C/I requires transport to R->stadion, C->airport or I->port. They can’t access these facilities if there is too much distance and/or too much traffic on the roads.
Also about maxing out buildings, I didn’t know about that before, but apparently the original SimCity did that too: The game calculates the city center and Commercials tend to max out in that city center, increasing population density, which in turn is a pre-requisite for Residentials to max out. They in turn require Land Value so make sure some parks are there or even better a lake, and not too much road pollution. Industry maxes out anywhere, but requires port connection and need access/be accessed by R/I.