Where can I watch Mushiking anime?
Mushiking was pretty much a large advertisement for a game and didn't get a great following - That isn't to say that anime based on games can't be popular (see Pokémon &amp; Yugioh) , but more often than not efforts to make a company's new product the new "thing" end in failure.

It didn't disappear, rather it faded away in time. A thing that happens to all media, unless it is significantly loved by consumers.

That being said, you can still find the DVDs in some places (Japanese):

Wow HD

Amazon Japan (Thanks @senshin)