I imagine that a number of routes- perhaps especially in the countryside- may not be suitable for electric vehicles yet. Add the burden of heat or AC on top of the vehicle's weight (plus packages) and it may be difficult to complete without recharging.

Since they are planning to be capable of retrofitting them, I wouldnt be surprised if they are planning on holding out a bit to see if battery and charging tech improves or standardizes further before investing heavily.

All of that said, I would be very surprised if they didn't electrify the entire fleet before 2030. The cost savings in maintenance alone would probably justify it even without environmental consideration.