Freight-wise, better load capacity can also be solved by ballastless track, using additional axles, or running longer trains. Passenger-wise, better stability can also be solved with canting - and wider tracks means significantly larger curves.
In return you get to buy significantly more expensive one-off trains and are unable to connect to your neighbors. Not exactly a great deal, is it?
Longer trains are not a simple solution as side tracks for passing by need to also be longer. Also longer trains means building and running more carriages, which is more expensive.
Why are wider inter-track requiring larger curves? It should be the same, but with better lateral stability.
Freight-wise, better load capacity can also be solved by ballastless track, using additional axles, or running longer trains. Passenger-wise, better stability can also be solved with canting - and wider tracks means significantly larger curves.
In return you get to buy significantly more expensive one-off trains and are unable to connect to your neighbors. Not exactly a great deal, is it?