content: If you add an additional 64 bits, that's 64 more lanes that need to be handled.  Those all require their own transistors in many places, as well as room for the wiring to sit.  That's only going to make the problem worse.  Plus, while it can technically handle more information with a larger word size, in many cases it just isn't going to help all that much, since the information you may be interested in for a particular instruction may well fit in much less than a full word (for instance, a text search algorithm).  In those cases, the extra word size is generally wasted.