What determines branchline coupler bandwidth?
Such couplers depend on lines of accurate electrical length ie quarter wavelength. So such designs are inherently narrowband, perhaps 15-20% fractional bandwidth. Visualise these working, by thinking the path the incident wave will take to anyother port via all possible routes. You will see that this sort of coupler essentially relies on interference and hence accurate phase shits. This sort of behaviour is narrowband.
Check out tapered coupled line directional couplers which are very broadband but bulky.
For truly ultra broadband but lossy performance, nothing beats a resistive Wheatstone bridge based directional coupler.