>>808631
Mecha started as a genre aimed at teenagers and children.
When the children grew up, they still loved mecha.
Which lead to the 80/90 young adult/adult mecha and sci-fi anime boom.
But mecha started to lose it's dominant role, especially in the teenager and children demographic.
Rumiko anime, DB, HnK, Slayers, the big things that young adult and teenagers watched in the late 80s to the early 90s.
Many of fans of these works turned later into otaku, which lead to late night anime continuing in this vein.
The switch to late night TV anime started in 1996, which is a time period where mecha as a genre wasn't even close to it's 70's and 80's popularity.
There were some standout hits (NGE), but it wasn't nearly enough.
Mecha as a whole was extremely late in the switch to the late night TV model. This cemented the slow death of the mecha genre.
Heck, the first big late night mecha hits are Code Geass R1 and Macross Frontier which both aired ~10 years after the switch to late night TV started.
And Code Geass R1 only aired in a late night slot because of bad planing.
R2 was moved to an afternoon slot, forcing Taniguchi to give up on his original vision for the series.
Also the anime industry started to collapse around 2007, which was particularly bad for the mecha genre.