The idea for doing a web framework for data scientists actually predates me finding out about reactive programming. And the dominant model was, and maybe in a lot of places still is callback based programming or event handling. It's very easy to understand how to code an event driven sort of way or using callbacks. But to get your callback logic right, as the number of events, the number of outputs and the number of intermediate sort of calculations as they increase, it's really hard to keep it correct.