A hop on, hop off tram system is much quicker and efficient than going into a subway station, then back up, etc. At least if you aren’t going entirely across town.
In the city I grew up in during rush hour subway trains leave less than every two and a half minutes and pack well over a thousand commuters each. That is an impossible feat for even the best tram system, as they're limited both in vehicle and station length, not to mention speed, as you can't have 40 tonnes of metal hurtling at 80km/h through intersections.
Whereas it is possible to have a subway where it’s completely impossible to get on the tracks without heavy equipment or the right keys.
There exist implementations of this, but it's essentially a train with extra steps.
[1] In my experience, at least in the cities where I have lived
I'd argue there are some limited places where classical trams on the road make sense, like some down town district. But you get almost the same utility with busses, passenger flow per lane is lower with busses, for a lot less money.
In towns with trams I've found Ubers almost always faster. That said, most American trams are at grade, so you get a bus that can't change lanes.
[1] Otherwise I have to check the Transit app on my phone and do some math factoring in the wait time and the time it takes me to get to the platform. The subway usually wins this contest too.