That's because everyone including Europeans spend plenty of their time watching the US election dramas so obviously it starts as early as possible because there's strong demand for it. It's a big business for everyone: the politicians, media, tech companies, the long line of hangers on, etc.

In Canada we know who the next 2 primary candidates were a year ago and elections not until next yr. That's almost always how it works, whether it's formalized or not.