Ultimately it is a decision by the channel provider as to what programs go into the OnDemand portal. You can always check what is supposed to be available on "spectrumondemand.com".
As to why a particular episode may be ommitted? There are a few reasons, but typically it is because the episode is available somewhere else. Specifically, on the Provider's own streaming site. This drives demand to their direct-to-consumer services. By making you go there for one episode, you're not likely to leave the site for the next one, or the rest or the series, or for the next series you want to watch, etc.