9x9 keeps people from being overwhelmed. Its the size of a chess baord, so its familiar to those people, and easier for teaching someone the basic life and death stuff. There are so many extra things you have to worry about on a 19x19, that it can just be a huge barrier to all but the most serious beginners.
Most importantly, it’s faster. 9x9 games can be played way faster than a 19x19. I can’t convince my friend to sit for 45min to an hour and a half while we play a 19x19, but a 10-15min 9x9 I can definitely pressure them into lol.
Recently bough an e-ink reader for this exact reason. I’m reading for the first time in almost 10 years and I am falling asleep way quicker and getting better sleep than i have in a loooong time. Best $100 ive spent in awhile.