Puncture resistant tyres, especially the Schwalbe marathon plus is very hard to remove and install - because they are so stiff. The thinking is, you won't need to do that very often given that you won't be getting punctures.

Assuming you're using the proper technique to remove one side of the bead and the tyre is completely deflated you can also try squeezing the tyre towards the middle of the rim - that may give you the extra space you need to easily remove the tyre. Or it might not. Depending on the rim (I'm not familiar with that specific wheelset).