Adoption curves show how the number of users for a product changes over time.
When something new comes along, it's not immediately picked up by everyone. Often, it's picked up first by a few people before being adopted industry-wide. Take cell phones, for example: as they gained in popularity, so did the network and support services. As newer generations started using them more & more, they were able to make use cases and benefits of it better than ever before.
Different sectors of a market come on board with new products or technologies for different reasons.