As the traffic to a service increases, design decisions made early become a more and more important factor affecting performance. You want your service to be scalable, to be able to handle a growing amount of traffic.

Having a proper REST web service implementation is one way of increasing scalability as it usually creates less traffic over network and less overhead without XML conversions needed in SOAP.

SOAP implementations can also be made scalable, but less of the architecture is explicitly designed for that purpose.