Probably because they don't improve cooling.
A top tier single tower (TRUE Spirit 140 Power) is as good or better than any of the twin towers out there.
Independent testing has shown NH-D14 is every bit as good and probably a little better cooling than NH-D15 when both are using same fans, and TRUE Spirit 140 Power gives 2-3c cooler temps than D14 on i7 [email protected]
It seems heatpipe cooling is close to it's maximum ability without some sort of new tech. The change from solid material heatsink to heatpipe heatsink made a huge difference in cooling .. and some types of heatpipes perform better than others but only marginally.
There were some vapor chamber coolers marketed but they suffered from problems like base not being flat. In 2013 I tested ID-Cooling FI-REEX Deluxe (got 3 coolers to get 1 with flat base .. I was able to lap 2 flat and 34d was warped too much to even lap flat). FI-REEX is a single tower 160x140x88mm (HxWxD) w/o fans, 113mm deep w/ 1x fan, 138mm deep w/ 2x fans. It has 6x 8mm vapor chamber pipes and 2x 'U' 8mm heatpipes with a rating of 300w. I found it cooled as well as TRUE Spirit 140 Power rated 360w. ID-Cooling has discontinued FI-REEX for a black colored version called Hunter VC-3D using 2x 140mm fans. I need to try getting new Hunter VC-3D and test it.