If you look at the weather briefing PM3 linked in post #176:
Cumulonimbus; Bases: 2,500 feet; Tops: 50,000 feet
50,000 feet is really, really quite high, even for an airplane. The drop altitude is supposed to be FL390, so 39,000 feet. Yes, you can fly above some weather. But other weather goes up so high that you can't and those thunderstorm clouds go up really high.
And the plane also has to climb to those 39,000 feet and they'd prefer not to fly a plane with a fuelled rocket strapped to it through possible lightning. Safety is the enemy of convenience.
ICON Launch Moved Back 24 Hours
James Cawley Posted on October 9, 2019
Due to weather in the area, NASA and Northrop Grumman have decided to move the Pegasus XL and ICON launch 24-hours to October 10 at 9:30 p.m., with takeoff of the Stargazer L-1011 at 8:32 p.m.
NASA’s live broadcast will begin tomorrow at 9:15 p.m. on
www.nasa.gov/live.
The teams are not working any issues. The rocket, airplane and spacecraft are ready to launch tomorrow. As always, safety of the crew and mission success are our main focus.
https://blogs.nasa.gov/icon/2019/10/09/icon-launch-moved-back-24-hours/
Teams Prepare for ICON Launch Tonight
Anna Heiney Posted on October 10, 2019
The Ionospheric Connection Explorer (ICON) will launch tonight on a Northrop Grumman Pegasus XL rocket from the company’s Stargazer L-1011 aircraft. The Stargazer will take off from the Cape Canaveral Air Force Station at 8:33 p.m. EDT.
The first launch attempt for ICON is 9:30 p.m. EDT. Follow live coverage here on the blog as well as on NASA TV and on the web at
http://nasa.gov/live beginning at 9:15 p.m. EDT.
https://blogs.nasa.gov/kennedy/2019/10/10/teams-prepare-for-icon-launch-tonight/
LD NET is quiet right now.
Pegasus GO for FTS internal power.
ICON ready for transition to internal power.
ICON is on internal power.
Countdown has gone very smoothly.
T-10 minutes. Weather is 80% go.