The 400 F5.6 sells for $1350 in Canada. To get a longer Canon lens you have to be ready to shell out $9690. That's a HUGE jump in price.... and I'd be willing to bet that there is a market for something in between, like a 600 F5.6.
A fixed lens is mechanicly simpler than a zoom lens. Since the optics are fixed they can be made much sharper than a zoom lens. They are lighter, more robust, and better sealed than a comparable zoom lens. To go to the 100-400 F5.6 zoom lens means paying out an additional $350 for a lens that will not perform as well... Yes you get the zoom feature, but honestly, aren't most of the shots you take with this lens at the 400mm end?