Who decides what mortgage servicer is to be utilized?

Servicers don't work for the borrowers, they work for the holders of the note (lender, investors which buy the notes from the lenders, etc).

A mortgage note will stipulate which is the servicer at the origination of the loan and under what conditions it may be changed and how. Usually, the initial servicer is the bank that originated the loan, but once the loan is funded and the bank sells it - the servicer may change. Even if the bank intends to never sell the loan, the bank may go out of business and its assets (including the loan) sold off to others (e.g.: First Republic bank recently), which may trigger a change of servicer.

Borrowers' opinions and preferences on the matter are irrelevant. They need to continue making payments, and once they get the notice of a change of a servicer they'll continue making payment to the new one. Their convenience is of no consequences to their obligation.