//why not?//
on the grown up railway, the TPWS system is fitted mostly to signals, and only a number of key points on the network where intervention is felt necessary approaching speed restrictions are fitted with the system. in most other situations it's up to the driver to drive according to the features of the line, and the conditions. drivers are expected to "know" their routes and regularly sign route cards confirming their knowledge.
Trams have no intervention system but then, neither do buses or other road going passenger vehicles. Trams are driven like your car, on a "drive by sight" regime, and the driver is expected to adjust speed to suit the geography and the conditions. Tram brakes are much more effective than railway brakes - they have to be when they are expected to mix with road traffic (as they are in several UK cities). But like the grown up railway, tram drivers are expected to "know" the routes they operate.