Bitcoin and 21 form the brand: the asset and the number that defines its scarcity. The .tech ending is a generic extension used by technology companies and developer projects, and it shifts the name's meaning towards code, hardware and infrastructure. A reader expects to find documentation, products and engineering here, which makes the address a natural fit for a company whose customers are other builders. The whole is short and pronounceable in any language.
Strategically, bitcoin21.tech covers research and development. A group holding the full bitcoin21 set, available together and spanning endings such as .exchange, .money and several countries, could place its engineering team, open-source work and API documentation here. Independently, it fits a wallet software company, a mining hardware designer, a node hosting service, a security audit firm or a technical training academy.