Hasn't home theater/tv manufacturer's given up (for the most part) on 3D home theater?
No more 3D on flat screen TVs. But 3D projectors are still being sold.
Why invest in the overpriced/underperforming tech if support from manufacturers is slowly disappearing?
How is it overpriced or underperforming?
In any case, the reason why someone would do it would be: Because they like to watch 3D movies.
The 3D extended editions of The Hobbit are exquisite. And Doctor Strange simply has to be seen in 3D. Watching Doctor Strange in 2D should be against the law.
And the 3D BluRay market is dwindling as well?
There is a recent trend towards only releasing 3D Blu-rays to foreign markets, but that just means finding an import from somewhere with the same region code as the US (or a region free copy), and buying from the Amazon site in that country.
These Marvel movies are not only available on 3D Blu-ray, but most versions of their 3D Blu-rays expand for all the IMAX scenes:
Guardians of the Galaxy
Captain America Civil War
Guardians of the Galaxy Vol. 2
Ant-Man and the Wasp
None of the 2D Blu-rays of those seven movies expand for the IMAX scenes.
Here are some other recent releases on 3D Blu-ray (imports included):
Spider-Man/Spiderverse (coming in April)
Aquaman (coming next week)
Fantastic Beasts The Crimes of Grindelwald
Dr. Seuss' The Grinch
Avengers Infinity War
Jurasic World Fallen Kingdom
Ready Player One
Pacific Rim Uprising
Tomb Raider (2018 version)
Jumanji Welcome to the Jungle
Star Wars 8
Blade Runner 2049
Valerian And The City Of A Thousand Planets
LEGO Ninjago Movie
Despicable Me 3
War for the Planet of the Apes
Transformers The Last Knight