Why do some universities require use of approved suppliers? Some universities in the UK require that all purchases from budget available to researchers happen through one or more authorized suppliers. They tend to be strict in this regard, even though some equipment might not be available or the price of it might be higher than elsewhere.

What is the rationale behind this policy? 

I fail to see any. If university where concerned with financial control then allowing outside purchase should make no difference, as they would still have to be approved by the administrative personal. If they are concerned with providing adequate IT support this might be done as well, just under a different contract with the supplier.
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.