Because the space is finite and widening the streets in the city center may be out of the question because one would need to raze down too many buildings people rather would want to keep. It is always a compromise between speed, cost and access to places. Plus Brits have it real easy, they have so much mothballed railway right of way so they can go all in with barriers and everything.
If trams must be on the main street where all the through going traffic also is, the "car party" would protest if the number of lanes was reduced or too many parking spaces were taken away. Most likely trams and buses would have shared lane and stops.
If, on the other hand, a narrower side street is used for the tram, perhaps because it is the primary shopping street, there still would be delivery vehicles and access to properties, people navigating to perpendicular streets, plus the odd lost hick, but all in all not too much to disturb the tram traffic if they are given the priority, which is easier to do when they are not on the main artery.
Priority at intersections anyway is the most important factor for journey time when the traffic levels are below jam formation.