Let's say, absent a boycott companies A B and C place bids of $1000, $1500, and $2000. Then they all decide to boycott a part from a country and the buy a replacement that's $100 more expensive. Then the buds become $1100 $1600 and $2100. Even though the cheapest bid still wins it could have been even cheaper absent the boycott.
I, and apparently most voters, don't want to pay that extra money.