Simple answer is that the REST API does not have all the functionality currently available in the OTA API. The REST API primarily focuses on simple CRUD operations of core entities in the client (CRUD = create, read, update, delete). You can compare the documentation for the two API's to see if you will be able to achieve everything you want with REST.
The main benefit of REST over OTA is that it does not require any client-side software to be installed/registered. You simply post HTML requests to a server and process the XML-formatted results. This opens the API to more clients than ever before.
The main benefit of OTA over REST is that it exposes more ALM functionality. For those programming in an environment that easily supports COM objects, you may also find the object-based nature of the API easier to work with, especially if your editor provides rich IntelliSense.
Basically OTA is the API of the past, and REST is the API of the future. It will just take time before HP can get all the functionality exposed via REST.