Because all the major operators (at least in Italy) really like cartels: if an operator increases the price of something, you can expect all the other ones to do exactly the same within a few months.

An example of this is when all the operators changed the billing cycle from one month to 28 days, adding another monthly fee in practice: even if only one operator started doing that (with outrage from the customers), now everyone does that, with the except of landlines where the regulator stepped in.