My guess on these reasons is that Amazon wanted to integrate MongoDB with their own proprietary algorithms, storage, infrastructure, and/or other solutions they

My guess on these reasons is that Amazon wanted to integrate MongoDB with their own proprietary algorithms, storage, infrastructure, and/or other solutions they already created. In my experience, databases (even with different structures) use a lot of similar components like transactions, caching, I/O pages, concurrency etcetera. If they rewrite the whole project, they could potentially use many parts that they already used in different projects.