Miroslav's comment 3.3:

At any point in time a client may have only one REGISTER packet outstanding, i.e. it has to wait for a REGACK packet before it can register another topic name.
[4.8 Topic Name and Topic Filter Registration Procedure]
This makes registration of multiple aliases synchronous hence time inefficient because every registration involves network roundtrip time and possibly retransmission timeout. If multiple parallel REGISTER flows were allowed, these timeouts would not sum up and I don’t think this complicates the gateway or client implementation much.
The synchronization of flows is especially harmful for sleeping clients (see 1.8).
(This comment of course applies to all the flows where this rule is stated in the specification, like PUBLISH with QoS > 0).