Maybe one box needs to communicate with a network of other boxes to keep traffic in sync over a wider area. I'm guessing that the bottom rack with the yellow cables plugged into it is some kind of networking gear.
Also, the equipment has to switch some fairly high-power loads - until the recent adoption of LED-based traffic lights, I'd guess that traffic light bulbs probably drew a few hundred watts each, and there can be many lights in a large intersection. You can see a row of 15A and 20A circuit breakers in the box in the photo, on the left side of the second rack from the top, with a 50A main breaker below them. The equipment below that rack looks like power-switching modules.
It's also entirely possible that this equipment is 20 or 30 years old, built at a time when computer hardware was much bulkier. There would be little incentive to replace it with something smaller if it was still working reliably.
In fact, this being repairable in the field by a person with a soldering iron and a bag of parts is a consideration.
Sometimes you cannot just get a full module to swap.
This. Repairability and durability are two reasons a lot of 'old tech' is in use.
I have a friend who is an engineer with PG&E who is great to take to parties full of tech nerds. They're always horrified by the '100 year old tech' running the grid. My take is that they're right to be horrified by the fragility of the grid, but have the 'why' exactly backwards - it isn't the vats of oil that make it fragile.
IIRC, in some cities (Chicago for one) there is a radio broadcast timing signal for traffic lights. There is a receiver in each control box. Sort of a WiFi NTP for traffic signals. Probably totally insecure and subject to hacking also.
Also, the equipment has to switch some fairly high-power loads - until the recent adoption of LED-based traffic lights, I'd guess that traffic light bulbs probably drew a few hundred watts each, and there can be many lights in a large intersection. You can see a row of 15A and 20A circuit breakers in the box in the photo, on the left side of the second rack from the top, with a 50A main breaker below them. The equipment below that rack looks like power-switching modules.
It's also entirely possible that this equipment is 20 or 30 years old, built at a time when computer hardware was much bulkier. There would be little incentive to replace it with something smaller if it was still working reliably.