Hi,
Its simple...
Transformer comes into picture when you talk about transmission.
Transfomer Induced voltage equation contains 4.44 factor.
E=4.44*f*T*phi
E -Induced emf per phase
T -number of turns
f -frequency
phi -maximum flux per pole
From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.
So always transmission voltage is multiple of 11.