The ability to navigate the road freely -- trolley buses often have trouble circumnavigating badly parked cars in their way, sometimes causing the driver to get out to reattach the "prongs" to the wires.
Also, trams are somewhat prone to "tram jams" caused by one faulty streetcar blocking the railway. And they simply cannot get around a badly parked car.
However I have seen trams broken down temporarily (a couple of times in the last 7 years of taking 2 trips daily) but these get resolved surprisingly quickly with minimal fuss. Additionally I've only seen a Trolley lose its connection a couple of times, and the resolution took about 1 minute.
This is of course heavily based on personal experience, if you have experience to the contrary I'd be curious where this is. A friend complained a bit about Trolleys in Wellington needing frequently reconnected, but I don't know how common that is or whether they just had some bad luck
That's not really the why, at least not here. The shortcomings of trams (including derailing and the inability to maneuver past stuck or badly parked cars) are handled by a 24h emergency crew dedicated for just that. I think they move on average less than one car per day, but those missions are much more frequent during wintertime.
Trams are much more prone to failure than buses.