The most common reason is - your maximum bid was not high enough to meet the minimum bid increment that eBay has set.
For example, let's say you scheduled a snipe with the maximum bid of $10.41 for an item that sells for $10 even.
You did not win the item because your maximum was not high enough to meet the minimum bid increment over the $10. In that price range, the minimum bid increment is $.50. So, your maximum would have had to have been at least $10.50, for you to either win the item, or get outbid again.
Not all eBay auctions end with an even dollar amount final price. In this example, the item would have probably been listed with an even dollar opening price - hopefully that makes sense.
Here are links to the different eBay sites explaining and showing minimum bid increments: