I guess too labor intensive, and probably problems scaling up to the size of current ships. Consider the last generation of sailing cargo ships before steam took over (https://en.wikipedia.org/wiki/Windjammer ), those were around 2000-5000 GRT, compared to a big container ship today at something like 170000 GRT. The tonnage difference is rather massive.

Also, if you look at pictures of windjammers, masts, rigging etc., would get in the way of cargo handling. Less so on a tanker compared to, say, a container ship, of course.

I don't know, but this looks very space saving and have much less moving parts.

maintenance. the materials used for actual sails stretches eventually and is less effective. presumably these towers have very little to replace.

They have a very expensive, hard to replace, and high maintenance bearing at the base of the tower that carries the full force of the sail leveraged by a long moment arm. This bearing (along with lack of height = low wind) has been the reason that Flettner sails and wind turbines haven't been economical in the past.