It's due to the Transpennine Route Upgrade related two track railway south of York. Hence not enough capacity for all services to run (particularly due to the single lead ladders at Church Fenton and Holgate Jn). IIRC that continues every Sunday for the remainder of the December timetable.
Whilst you could bus to Church Fenton, most people would prefer a single change option. Church Fenton's narrow approach road and small car park also isn't suitable for anything more than minibuses, but most of these kinds of jobs end up being done by coaches.