After 9/11, President offered Speaker of House use of military aircraft travel for security (continuity of government) reasons.
When Pelosi became speaker the first time, she demanded to use the C-32 (200 seat Boeing 757) vs the 12-seat Gulfstream that previous speaker had used…but ultimately lost that battle (not sure what she's been using this year)
There is no 'requirement' for speaker to travel on milair, it's just been an (expensive) perk since 9/11
https://www.cbsnews.com/news/pelosis-speaker-shuttle-the-inside-story/