Every situation is different, and the reasons for not sharing names change depending on the the stage of the discussions.
For example, if discussions are advanced enough to involve negotiations on valuation, in general, you want potential investors to compete, not collude.
Unless you know the parties involved well enough to know that they will not join forces to work against you in negotiating valuation and/or governance, it is not a good idea to share the names of other VCs you are talking to.