Similar to the real world, having a majority (or all!) of your fleet with a single manufacturer should provide a discount for further orders or leases, and the discount is instantly lost if you make a purchase with a competitor.
For e.g., you build an all-Airbus fleet for 20 years, and get a decent discount to buy more Airbuses on your fleet renewal. But then you buy Embraers once, and Airbus gets upset and reduces or removes your discount.