But the angels had just used their teleportation getting Kevin in the room, and in the Becky episode Crowley was able to teleport in and out of Dick's car without problem. If the leviathans have some sort of power nullifier, not only has it never been mentioned, it's not applied consistently. Hell, why would leviathans be given power over angels and demons to begin with? They didn't exist when the leviathans were made.
And then there's the issues of why God would leave angels, which are apparently powerless against leviathans, to guard a tablet about how to kill leviathans. Surely he must have seen the issue with that, so why not assign something else, or give angels the power to fight leviathans? And yeah, leviathans are the oldest known creatures in the supernatural universe barring Death and God, so why would they be vulnerable to the bone and blood of creatures created much later?
It just seems like the writers haven't put much thought into the leviathan arc and are trying to force how dangerous they are on the audience instead of really showing it.