Why do some universities require use of approved suppliers?
Some universities can get a better deal from a supplier by promising them to buy exclusively, or to buy sufficiently much from them
It is easier for the accounting department to deal with the same provider, since they can re-use the same documents.
Dealing with a new provider has a certain degree of risk, including the quality of products, the timeliness of delivery, etc. If you deal with the same provider, you acquire enough information to feel yourself in control.