Usually because the lane is only being shut for a few hours, and it would cost more and take more work to set up a portable traffic signal. In Ontario, if the roadworks take more than a day, there's usually a traffic signal set up.

Interesting anecdote: I was driving through North Dakota this summer and hit resurfacing roadworks. It was the first time I've ever seen it, but rather than setting up hundreds of cones and directing traffic through them, then re-arranging the cones to do the other lane, they just had a flagman on each side and a pilot vehicle guiding 10–20 vehicles at a time through the work zone. No cones at all. I'm sure that project was completed in half the usual time.