re: Why isnt AAC(Autoclaved Aerated Concrete) used more in construction?

This.

Any new technology in construction has to go through a tedious vetting process with a great deal of pushback from the status quo.

I was a commercial sales manager for a very large ready mixed concrete producer. It took many years AFTER years of R&D to persuade Architects and Structural Engineers to allow new products as options.....much less to have them actually spec the product.

I worked for several years along side a company out of Vancouver on their innovative foundation waterproofing product. While all Structural Engineers show great interest in innovation, they are also all reluctant to be the "first" to approve it on their project. There has to be years of research and dozens or more of example projects available for them to even consider actually allowing the product. After years of a concerted effort with the manufacturer, we finally got the product listed as an option on the Georgia Aquarium project. And it was only to be an option on small areas scattered around the project. That was around 10 years ago and the minimal use on that project led to it being used on future projects.

Same exact process occurred in working to get structural fiber specified as an alternative to traditional concrete reinforcement on elevated mezzanine slabs and industrial slabs on grade. Worked with the innovators of that technology out of Pennsylvania pitching it to SEs for many years before it was finally specified and used on the 12th and Midtown project.

Aside from those, we had our own in house R&D on SCC ( self consolidating concrete) and Pervious Concrete that our Tech department spent years designing and testing.

Of course, the traditional waterproofing and steel reinforcement producers did not like it and fought tooth and nail to maintain status quo. We were adding the waterproofing and reinforcement ( other than around columns and beams)to the concrete mix and that was detrimental to their industry. It required innovation and progress on their part and up and down the line.....which means investment.

I imagine that the same dynamics are at play in this situation.