FRET only works for interactions between 1nm to 20nm.
To get an elaborate answer on this one, I guess you should ask at physics. Very basically, FRET works by that an excited chromophore transfers energy to a nearby chromophore, through dipole-dipole coupling, which in turn emits energy (light) which can be detected. This is a quantum mechanic effect, i.e. these are very small energies. The emission energy between from the donor to the acceptor chromophore is inversely proportional to the distance between to the power of six according to the equation:



Where r is the distance and R0 the Förster distance between the donor/acceptor-pair (distance which the energy transfer efficiency is 50 %).This means that the energy to be detected very rapidly decreases. The Förster distance is in the range 2-10 nm for different chromophore pairs, which leads to the upper limit of FRET use.

The lower limit I don't know if it's due to steric effects or some quantum mechanical effect I do not know about. But since proteins are in the size range of the lower limit they must be in really close vicinity in order for their interaction to be detected using FRET.