workmanship and also cost-profit, for bulk purchase they able to offer you lower price when you give them certain amount of orders, this will reduce their transport charges and material costs and etc...

most important is you need to see their workmanship, reviews and make sure they do it according to the specs given by your condo management

sometime those recommend by people in owner group is also a trap, they just simply throw out bulk purchase and who knows maybe they also get a certain commission

if your condo management has a in-house contractor then just go for them, at least it is recommended by management and save you a lot of hassle and dont need to worry them running away