Why are some electrical systems only two phases of a three phase system?
Phase balance is handled upstream from the meter socket

You will see this in two places:


Densely developed areas where the low voltage grid is fed from multiple transformers at once (something called "network service" in distribution parlance)
Larger multi-tenant buildings that exceed the capacity of a single split-phase service drop from the utility and are not configured to allow for per-tenant utility drops


Either way, what will happen is that the buildings or tenant spaces will be connected to 2 of the 3 phases to get a 3-wire service from the 4-wire wye system, but the phases they get will be rotated/transposed to balance the load:


Building/tenant 1 gets A and B phases
Building/tenant 2 gets B and C phases
Building/tenant 3 gets A and C phases
Building/tenant 4 also gets A and B phases
...and so on (there are other ways to arrange them, this is just a demonstration example).


This way, you get an evenly balanced load from several 3-wire services on a 4-wire system.  Furthermore, a similar pattern occurs if you go down a 3-phase panel on a 208Y/120 system plugging 2-pole breakers into one side of the panel:


Load 1 gets A and B phases
Load 2 gets A and C phases (really C and A, but it's irrelevant)
Load 3 gets B and C phases
Load 4 gets A and B phases again
...and the pattern continues from here


As to implementing this:


If these are individual buildings, then the utility will implement this at where the service drop/lateral from the utility splices into their low voltage main (quadplex)
If these are tenant spaces in a multi-tenant building, then there will be facilities in the meter bases to control which phase busses the meter sockets connect to (often times, a pair of movable links are used for this, but changing them will require electrician help and a utility power cut to the whole building as they're upstream of the tenant meters)