This is an interesting question, and since I just got out of a biology lab final, I thought I'd add my ideas...
For one thing, they're too mushy... just about everything on land has a skeleton of some sort. Insects have exoskeletons, while vertebrates have endoskeletons. Most molluscs have shells which are kind of like exoskeletons... but octopi have lost these. The nautilus still have them but I can't imagine them as a land animal. Slugs manage to exist without an exoskeleton, and are molluscs. However, they have to remain small and don't have much room for tentacles/a brain/ etc.
For another thing, cephalopods just work really well in water. They are products of a very long line of evolution, perfecting them for a marine life. They can move very fast, have very dexterous tentacles, advanced eyes, large brains, ink, etc... if they left for land, none of these things would be able to exist in the current form. So, they'd end up back to the level of slugs, probably.
There's already a lot of competition on land... mammals, insects, reptiles, etc, have pretty much taken up every ecological niche possible. For a cephalopod to thrive on land it would have to be able to compete with these. They'd be much slower on land, barely able to survive under their own weight... which doesn't make for a good predator. And it isn't likely they'd be able to become herbivores. (the same basic idea has been applied to why insects aren't found in marine areas.. there simply isn't anywhere for them to go, ecosystem-wise). Anyway, if a mass extinction killed most chordates on land, perhaps the cephalopods would have a chance. But until then...