Many answers relate to the competitive aspect, but this really applies equally to all FOSS network-based projects or not. A distinctive feature of websites is that users in general do not have access to the code in any form - source or binary, so the typical FOSS requirement that you make the source available to anyone who receives the binary code becomes almost meaningless. I believe this is a major obstacle to the widespread use of FOSS licenses in conjunction with websites.
(Recently, there have been attempts to develop FOSS web-oriented licenses that specifically address this problem, but so far no one has caught them.)