Palm was the market leader, it would have been the obvious choice. Palm had been around since 1996 and by 1998 had sold 30 million devices [1]. PocketPC didn’t come out until 2000, in 2001 they had only sold 1.25 million devices, equating to less than 10% market share [2]. From what I remember Palm Pilots were the go to choice for PDAs, they were simple and worked. Other devices had come and gone. It would have been odd if they chosen something else. I doubt anyone was thinking it would be used for 20 years, though I don’t think people would have thought it would go away at the time.
[1] https://history-computer.com/palm-pilot-guide/ [2] https://www.zdnet.com/article/pocket-pc-sales-1-million-and-...