The short answer is: to save memory.
The cars and replay seem to be a case of damned-if-we-do, damned-if-we-don't. If we don't stop the cars and reverse them in replay, we get piles of bug reports. If we do try to replay the traffic, we get bug reports too.
The current implementation is a bit strange: when you replay traffic, the cars will go back a bit in time, but at some point they will just stop and refuse to reverse any more. What's going on?
The answer is that the cars have the memory of a goldfish. They simply don't remember where they came from. Each car knows what "link" it is on, and about when it got onto that link and how fast it is going. (A link is a single straight piece of road.) So when we go into replay, we can easily move the cars along their links as time goes forward and backward.
But when we reach a time earlier than when the car entered the current link, the car has know idea how it got there, so it is forced to stop.
This is a simple case of not wanting to burn four tons of memory on a feature that is mainly visual. To replay the cars, we would have to accumulate a history of every link a car has been on as it drives. For 20,000 cars and a sim that's been running a while, that's a lot of memory to burn just in case you happen to hit the replay button.
In fact it gets worse. The cars are kept in a data structure that tells us who needs to make a driving decision and when.* This structure is optimized for the cars moving forward in time. We'd have to make and maintain an entire second copy of this structure to move the cars backward; again burning CPU and memory while you fly just in case you might hit a replay.
So instead we just provide replay on the current link.
* Programming nerds: the cars are in a priority queue by time to next navigation decision. I consider this to be very clever.