Because when you are dealing with corporates the amount of time needed can vary massively by customer. A bank or healthcare firm will come at you with endless feature requests, support requests etc. over things they claim is a regulatory requirement / they must have ASAP.
You also might offer the first bank a discount because once you have one onboard its much easier to sell your product to others. They know a competitor has already done all the due diligence and decided you're safe, so the risk they spend months evaluating you to not be able to move forward is minimal.
I concede that I mainly approach this from a non-enterprise mindset, but even trying to research these things for enterprise can be frustrating. It does seem to make sense for products that are at a large scale, both in deployment "size" or effect and pricing.
You also might offer the first bank a discount because once you have one onboard its much easier to sell your product to others. They know a competitor has already done all the due diligence and decided you're safe, so the risk they spend months evaluating you to not be able to move forward is minimal.