Many charities and not-for-profit organisations have complex staffing and payroll structures. Most companies will employ a fairly stable number of staff either part-time or full time, with staff arrivals and departures being relatively infrequent.
Charities, on the other hand, may have a large bank of staff to call on for short periods at random times of the year, so that only a small percentage of staff on file might actually receive a payslip in any particular month.
In addition, many charity staff members are volunteers who may only receive expenses payments.
Where staff are paid a wage, this can often be on a short-term contract basis, perhaps for just a few weeks. Other staff may be paid on commission only, with zero base wage. And there is also a generally high turnover of staff, resulting in a fair amount of admin in getting new staff added to the payroll and then removed again.
As a result, many such organisations find it easier to rely on expert payroll outsourcing to ease the burden on their internal teams.