I imagine building such a spacecraft is a relatively large undertaking. More centrifugal force means a larger diameter spacecraft and a force that is trying to

I imagine building such a spacecraft is a relatively large undertaking. More centrifugal force means a larger diameter spacecraft and a force that is trying to rip it apart (that's what's generating the gravity after all). With NASA's current budget, they probably figure it's easier to just deal with microgravity for a spaceflight with a limited duration (like to Mars). Generating gravity only really makes sense for spaceflights of longer/unlimited duration (interstellar travel).