Competitive bidding was prohibited for quite some time due to the following reasons:

Consulting jobs, unlike industrial and construction work, are not suitable for precise cost estimates and hence precise bids.

Here competitive bidding,  would  encourage  cutting  safety and  quality,  in case  of lower bids and padding/over designing in the case of higher bids.

Later, Competitive bidding was approved by Courts of law on the reasoning that free trade is restrained in an unfair manner.