The problem is that this mandates the data be recorded -- more specific locations of phones that are in use, for example. Right now the cell companies log your calls and their times, and I presume they log traffic levels on their cell towers. I don't know if they even log what cell towers you used, but I don't believe so and I don't believe they keep a more detailed track log of their E911 positioning on you. (With many phones, they only get that when the phone is making a call, which is good.)

Doing this would require they record much more data, and in turn would mean that data could be found by others and used against you. People do not want a complete log recorded of where they have traveled. Nor do they want their cars recording such things.

Now such recordings are always a two-edged sword. Sometimes they can prove you are innocent, but sometimes your own car or cell company would be turning against you. People don't want technology that can betray them.

Every cell tower is in use, just knowing how many people were using a tower near the accident at a given time is of no value.