Why not extend Downtown Line to join North-South Line?

The North-South Line is one of the most used MRT lines and is always packed during peak hours.

I had a chance to take a bus from Hillview to Woodlands, and went past the new Downtown Line.

What struck me was that the distance from Bukit Panjang to Woodlands was perhaps no more than 5km. Why did the planners not link the Downtown Line to Marsiling or Kranji station on the North-South Line, given that the distance is not too far?

Given the relative undevelopment in the area, extending the Downtown Line to join the North-South Line would have been relatively easy. It could also ease the congestion on the North-South Line instead of commuters having to wait for the Thomson Line to be ready.

Foong Swee Kong