That's because some companies may bid so low that there's really no profit to speak of and then try to do shoddy work and use inferior materials once they get the job. That would certainly affect the quality of construction, and the public would be the victims.
有的人為了標到工程，不惜把價錢壓得很低，低到根本無利潤可言；一旦得標，到時候再來偷工減料，免得虧本，這種作法定會影響工程品質，結果所有民眾成了受害人。