I believe these others keppra, depakote, tegretol and other new ones aren't used in K9's due to their short half life and a K9's high metabolism the drug doesn't hang around the body long enough to do much good. And i know Keppra is expensive, depakote & tegretol, not so much.
Oh and Felbatol I think causes something bad in some humans, i think some form of serious Anemia.
It is still used because, again for some people, it works very well as an anti-epileptic.
Depakote - Enteric coated tables, works in the intestines, but again does digest quicker than us and the enteric caoting delays it too long to dissolve.
Depakene - gel capsule/let - dissolves in the stomach. It can be harsh on some sensitive stomachs.
Diazepam - I believe is used more to break the cycle of the seizures, Via oral tabs, or rectal suppositories, even the rectal gel (but these are very expensive)