I presume the delivery schedule was baked-in to the contract, as Stadler don't have extensive storage facilites near their factories so they need to clear what storage they do have for other customer's units that are coming off the line.
It was probably based on the assumption that there would be minimal issues post-delivery and introduction, which would have allowed the withdrawal of the 507 & 508 fleets to be accelerated. Instead, they have needed to be kept to the end of their leases, which has meant there is not enough room for the units being delivered at the depots so they were being kept at Walton and are now going to Long Marston. All of this was presumably exacerbated by the delayed introduction into service due to COVID and not sorting out the agreements with the unions about how they would be operated ahead of time (that really should have been done during the specification stage, not after units had started to be delivered).
It's interesting to see that Nexus have already postponed the introduction of the 555s until at least next year, after unspecified issues were found during testing on the Metro network. I think they own their existing Metrocar fleet though, so they are not in the same position as Merseyrail who, stuck between a rock and a hard place, seem to have made the choice to soldier on with the introduction of unreliable units rather than having to pay for extending the lease on the existing stock and potentially put them through deep maintanance (it's arguable that the contract with Stadler could have been written so that they bore those costs if the introduction of the new fleet did not go as planned).
As I said earlier in the thread, I think a lot of these issues are due to Merseytravel having no previous experience in the aspects of procuring, testing and introducing a new fleet. In my line of work, post-delivery faults of the scale seen with the 777s would cause the programme to be halted and there would be contracted daily penalty payments to the customer until solutions were available (which is a great incentive to get as much testing, based on real-world use cases and conditions, done as possible pre-delivery).