This is the recurrent concern in every company. There is of course not a unique reason but a global context that lead to this situation
Reason#2：Not enough time / Product Life Cycle is getting shorter
processed during the product development by engineering, sourcing and PM need to be covered within a shorter leadtime. One of the solution to catch the milestones will be to reduce the number of vendors thus reducing the workload of the sourcing activity.
The consequence will be less negotiation leverage on the cost and on the procurement conditions, or say, different negotiation leverage which need you to adopt new methods, make new analysis. For example, change from negotiating volume to volume consolidation across vendors.