I manage volunteers and I think you'd need to go further in depth to see what actually is reasonable/unreasonable, which you probably don't want to as it would be outing.
For me, if I'd asked you to sort it, then I'd have tried to find what the issue was and if it could be sorted. I'd have discussed with you before asking someone else to take over for sure.
Sometimes volunteers run out of enthusiasm or energy to complete things and it can be tricky to work out whether they need a little push to get the enthusiasm back, or whether they really want to step down because they don't want to do it any more.
Sometimes you do have the situation that they don't want to do it any more, but they don't want someone else to do it either. That one can be quite tricky!
It takes a bit of tact and you don't always get it right even after a lot of talking and doing your best.
It may be that you told them exactly what was needed, and it was never produced, then they got put out because it wasn't going anywhere which wasn't your fault - but they perceived it was. Maybe even they thought they'd given you permission to buy what was needed/recruit more people, and you didn't realise that. I've been in that situation on the receiving end before.
Or it could be that you skirted round the issue of spending money (knowing there wasn't a lot spare) and more volunteers (knowing everyone was busy), and they got the impression it was just more time you needed. They gave you more time then assumed you had got bored and weren't going to do anything else.
Or maybe you presented it as a lot more money needed than they had spare, but then they saw it on offer/someone offered it cheap or free or to pay for it, so that's why it moved forwards.
Again I've been there where something has looked impossibly expensive then it's popped up on "free stuff on Facebook" or someone has offered to donate it.
Whatever happened they should have talked to you more before asking FT to oversee.
What I'd suggest is speak to the manager. Say that you were really enjoying it, and you'd felt really pleased to have your own project and you're disappointed to now be in the position of "asked to join the team" for something you did most of the donkey work.
But it may be that only being there two mornings they do feel that it will need to be a team effort and they need someone more full time to run it. Unless you talk to them, they won't know how you feel, and you won't know why they have done this.