Because FastAPI is much faster* than both Flask and Django (see https://www.techempower.com/benchmarks/#section=data-r19&hw=...), supports Python type hints, is a great fit for my target architecture (SPA), supports both RESTful and GraphQL API styles, offers built-in validation and much more (https://fastapi.tiangolo.com/features).

*) Having said that, from the performance perspective, .NET Core and Julia (from my options) are certainly much more tempting.