Uranium is less than 7KB when zipped. This makes it perfect for mobile devices, where minimizing KB is essential.
Uranium makes the view (your UI/UX) rely on the model (your HTML). The declarative aspect is not designed for performing functions. You wouldn't want to add attributes to an element to perform some JS logic (this is exactly why "onclicks" should be avoided) — that's what events/listeners/callbacks are for.