Why does a decrease in Arctic sea ice extent and Eurasian snow cover weaken the Jet stream?

If someone suggests such a connection, it is up to them to present data to support their hypothesis. I don't know what you mean by "weaken the Jet stream".

AFAIK the jet stream is powered by electric current from the sun, which may or may not correspond to local weather patterns. For example, if the jet stream moves south, weather gets colder in parts of the US and that is why weather in America is so much more variable than weather in some other countries.