Not trying to beat a dead horse here, but 1/10 electric is not the same as 1/8 electric. It ain't even close. Just because people have longer mains in 1/10 does not mean they are going to apply to 1/8.
Running 20 minutes or 30 minutes in a 1/10 is nothing like running 20 minutes in a 1/8. The technology for 1/8 electric has improved and it is close but it is not there yet for 1/8 scale electrics to reliably run 20 minutes regardless of track conditions and esc/motor setup. Most of the guys I know with race spec 1/8's loaded up with high dollar Neu motors, ESCs, etc still have to constantly watch temps and battery drain because the voltage/amps required of these setups is immensely more demanding than 1/10.
It is possible to run 20 minutes, even 25 minutes but it CANNOT BE DONE CONSISTENTLY without having a low KV/high voltage set up which means spending a ton of money on 5s & 6s batteries. This is why shorter mains are necessary. Simply going from a high grip track to a somewhat loose track with a lot of wheel spin could drop your runtime 10 minutes. As I mentioned, I can run 30 minutes on one track and can't get 20 minutes on another with the same setup simply because the track conditions changed.
The rules have to be set so that Joe Racer doesn't have to buy five different motor and battery combos just so he can be sure to get 20 minutes regardless of the track he is visiting.
The main runtime has to be achievable if Joe Racer is running a $60 dollar feiago and a $70 zippy lipo or a $250 Neu motor with a $250 Flightpower lipo. Requiring longer mains will raise cost and turn away people.
As the technology and more importantly, the reliability of the ESC and motors improves, we can start exploring longer mains. But right now, we need to focus on just showcasing the cars superior performance over the nitros in 15 minute or less mains. Clearing 30-40 foot triples with seven feet of run up and blistering acceleration will more than make up for not running a 30 minute main in most people's eyes.