I think it's just that the bandwidth of the microwaves is not much by today's standards. It was capable of 150,000 simultaneous telephone calls, which is someth

I think it's just that the bandwidth of the microwaves is not much by today's standards. It was capable of 150,000 simultaneous telephone calls, which is something like 10Gbs. It may be that modern numbers were higher, but possibly not much as you have to share the microwave spectrum with other uses. In a fibre you get the whole optical spectrum to yourself, so you can achieve much higher speeds. You can also run many fibres in the same ducting and increase bandwidth that way. For backup / resiliency you just run more fibres via different routes.