I discussed this in a "how to post" here.
But from the specification standpoint, the bounds of desired or expected behavior during a bulk transaction makes it a very risky proposition to include in an add only spec. The concern is a bulk operation of the same type, and a transactional operation of multiple types would have very different concerns on specific endpoints. However, both of these would have the need to define a way to respond to failed nodes within the operation. A bulk load might want a report of specific items which failed, while a transaction would require all proceeding operations to be undone, and would require implementers and designers to understand and more importantly support the concepts of transactional isolation, which is not a base property of a restful service.
Under the mantra of "be liberal in what you accept and conservative in what you send", the current method allows for enough flexibility where a bulk operator can define a resource like the one I linked, as well as implement transaction isolation for heterogenous bulk operations.