If it needs to be done urgently, then there may be uncertainty in the estimates (incomplete scope of work). A fixed price contract will likely inflate the cost so cover contingencies. Cost plus ensures the contractor will have all their costs covered so the risk to contractors is low vs. the risk to government is high.
If it needs to be done immediately, the government may just want to assume the risk of cost overruns so they can get contractors on board sooner. Saving Changes...
