I have been giving the same reply to this question over quite a...

I have been giving the same reply to this question over quite a few years.
There are a few reasons.

The main one is that there is no actual demand for lower CO2 emissions concrete, lots of talking but almost nobody is willing to pay extra, design engineers are not willing to take risks and use it in the specifications.

The second one is that the results of the study funded by ADN on using HRM in concrete have shown very limited benefits, achieving 7% reduction in CO2 emissions in a narrow range of concrete mixes. It was a good and professionally done study. The small reduction in CO2 emissions was achieved without increasing the cost as well.
The reality is that there are other methods of achieving much higher reductions with other additives which are already existing in the market and are distributed by suppliers to concrete producers.
There is concrete with 30%, 50% or more available today.