Were they identical plants? Sometimes I find dipladenias mislabelled as mandevilla - and there are a number of landscaping ones now that don't climb at all.
Otherwise, and if that is a "now" photo of the non-climber, perhaps the newness of it's planting and the fact that it seems to be putting its strength into flowering. You might need to remove the flowers to encourage growth, and that seems a great shame as it's doing beautifully, if only at a low level!

There are some Mandevillas that are shrub form. I had one and it stayed a nice globe shape.
Need to buy one that is already twining up a support to make sure it is a climber.