The majority of salaried people are paid monthly. Bills and mortgages are paid monthly and many find the adjustment to a four-weekly income difficult to manage when the dates for standing orders and direct debits are fixed. Whilst younger pensioners may cope, older ones may not be as adept at moving money around to ensure that there is enough for bills at the correct time.
It would involve the simple addition of a tick box on the pension pay systems to add monthly pay as an option. Once opted, it would not need to be changed.