Answer:
One of the main reasons is cost.
Step-by-step explanation:
The previous or old setup of normal cable which is there is vast and spread all over and it is not a one-day task to remove all cables at once and replace them. The cost of optic fiber is also very high compared to that of normal cables. So they need to change the connection in stages so that the connection remains intact. The upgrade is going on without harming the connection slowly in stages.