The reason is retention. For a company that sells a service where they pay a single overhead (like maintaining a structure) it always makes more sense to lose a little money and retain a customer if prices are going down elsewhere.

That is to say if your internet plan is $80 and they have intel that a local competitor has started selling a similar plan for $60, it makes more sense to spend 3 minutes talking to an existing customer about lowering their bill to $60 rather than let that customer discover a cheaper plan and switch to someone else. If they let that customer switch they lose the whole $80 whereas if they just lower that customer's price they only lose $20.