Money.
For a Continental level team (3rd Division – possible invite to races like the TOB and others in Europe), with up to 10 riders a bike manufacturer will pay around €500,000 plus the providing of equipment per season.
A ProTour team, such as Columbia HTC, you'll be talking around and over €1,000,000 plus around 100 or so bikes, hence why in the road scene the big teams ride the big brands.