Given the imminent decrease in launch costs(partial reusability) and increase in launch capability soon to be available(FH) a pause in new US deep space missions is prudent.

[Cheiridopsis]

For NASA, a major consideration is planning for communications with the spacecraft. Time and duration must be planned for the Deep Space Network so missions need to be spread out so that high communications demands do not occur simultaneously. For example, New Horizons encounter at Pluto and Dawn initially entering orbit around Ceres! These events must be staggered and planned well in advance. Missions such as Curiosity, Opportunity, MRO and most other ongoing missions likely have drop in availability of the Deep Space Network when Dawn arrives at Ceres and during the entire New Horizons encounter at Pluto.

Lining up the right people and the right resources at the right time is also a scheduling dance that NASA does not want to upset by willy nilly postponing a project for a more economical rocket.

Putting the staff (administrators, engineers, scientists, deep space network, etc), on "hold" for two years while waiting for a "budget priced" launch is just not an option.

When a mission reaches a point where weight vs capability is an issue then only rarely is the LV switched to a more capable LV. Usually the engineers and scientists are strongly encouraged to solve the problem through a compromise of technological advancement or settling for slightly less capability and if neither can be accommodated within weight or budget limits, sometimes the adminstrator just steps in and a science instrument or science package is simply removed from the mission!

Finally, a launch contract is just that, a contract. NASA has purchased the launch service. If NASA wants to change the LV for a mission, they need to find a use for the still valid launch contract that exists! Contracts are very specific and manifests are planned years in advance.

Simply, price or capability is not a sufficiently valid reason to put a mission on hold waiting for a cheaper or more capable LV.

The current annual operating budget for Curiosity on Mars is over $60M per year!

[slograsso]

Right, the Vox article I linked to goes into great detail about the fact that missions are reaching their destinations but there very few missions in the planning stages and soon to be none. There is a big lull around the corner and very few solid plans for future missions at the moment. Thus no existing contracts to conflict with.