There's always a reason that a bike is for sale.

With a new bike from a dealer, the reason is that that's what they do for a living.

In the case of a used bike from an individual, there are many possible reasons. Some are good from the buyer's standpoint (seller needs money more than he needs a bike, bike doesn't fit seller, seller is changing to a different type of riding, seller simply wants a better bike, etc.). And some are very bad (seller is trying to unload a piece of junk). It's pretty tough to figure out the reason when your only contact is by email, and you can't inspect the bike.