Why might management place pressure on purchasing agents to buy from the lowest bidder?

Because on paper it shows a lower immediate cost rather then longer term savings and potential quality issues. Getting a 2 million dollar bid rather then 4 million shows an immediate savings in contrast of 2 million. Depending on how budgets are controlled they may have been given 4 million and any saved may be discretionary in their use so cutting 2 million could mean 2 million can be used towards another project that may be running over budget. In addition many incentive programs provide bonuses for come in under budget and thus, saving 2 million could result in a bonus for management.