There are a lot of things, like say your competitors eating your lunch. If someone comes along and sort of satisfies the need it's easier for them to get people to upgrade than it is for you to come along later with a more polished product and ask them to migrate.
Depends on how much lock in your product has. Often an early releaser ends up creating a market and then a competitor comes in and releases a much more polished product and everyone jumps ship.
