>>1200882>all the street here are served by bus routes. If you need to convert bus line to tram then rails would need to be laid on basically all roads. At that point why not simply use buses?
The point of rails is to have the same transit service offer higher capacity/throughput than with buses. Maybe not on all streets, but on some.
If you need to increase transit capacity, what are the options? One, building new heavy rail lines. London, for instance, has a dense subway and railway network, and building more heavy rail into built up areas with rail service can be expensive and complex. So, if heavy rail isn't an option, the next step can be light rail. You replace bus routes with tram routes, and immediately double or triple capacity.
Dense bus networks tend to be relatively spread out (on the street grid) because each street can only handle so many buses. It's more efficient to have fewer but rail-based trunk lines instead.