Originally Posted by est-gratuite

Officially it's for "engineering works" (even though it's a brand new line).
In reality the vastly reduced passenger traffic on weekends in that far east end of London means they can save money by just not having any trains, train drivers and station staff working. Also, weekend journeys on public transport in London cost less for passengers, so TFL (transport for London) make a lot less money. It's a pure cost cutting measure.

Don't get caught out.