Because that's what she was thinking about at the time
At the beginning of the episode, there's this:
CRUSHER: Dalen, I'm sorry to hear about Patricia.
QUAICE: She'd been ill for some time.
CRUSHER: Is her death the reason you're giving up your post here?
QUAICE: We had a lifetime together, doing all the things we'd ever dreamed, and more. But when she was gone, I couldn't continue to work in that office, sleep in that bed without her. The absence of her was too distracting. I'm not sure that I'm making any sense.
CRUSHER: Jack and I didn't have a lifetime together, only a few short years, but I understand. When you realise someone you love is lost forever
QUAICE: You know what the worst part of growing old is? So many of the people you've known all your life are gone and you realise you didn't take the time to appreciate them while you still could. Oh, I'm sorry. There was no reason to heap all this emotional baggage on you. I usually travel light.
Star Trek: The Next Generation 4x05 "Remember Me"
The very next scene is the one in which Crusher is captured by the warp bubble:
WESLEY: I just need a couple more minutes. I'm ready to try the new warp field. Mom?
CRUSHER: Don't let me interrupt.
LAFORGE: Wesley!
WESLEY: Right! Okay!
(There's a sudden flash. Wesley looks concerned.)
[...]
WESLEY: That shouldn't have happened. Why would there be any visible phenomena outside the drive?
RIKER [OC]: Prepare for umbilical disconnect.
LAFORGE: Are you done?
WESLEY: Yeah. Mom?
(But she's not there)
Later, we learn from the Traveller [sic] that this is significant:
PICARD: Is she alive?
TRAVELLER: As long as she thinks she is alive, she is alive.
RIKER: What the hell does that mean?
TRAVELLER: Your species have very narrow perceptions of time and space and thought. When Beverly Crusher was caught in the static warp bubble, she created her own reality. Her thoughts at the precise moment she was trapped determined its shape and form.
Crusher even comments on this:
CRUSHER: The Traveller used his thoughts to alter warp fields. Thoughts became reality. Now I'm in a warp field. Could my thoughts have changed this reality? Come on, Beverly! What's the next step? What was I thinking at the moment Wesley's bubble formed? Dalen Quaice. He said all the people he'd known were gone. I thought of Jack, I went to see Wesley, the flash in Engineering. That's when it started. That's when I started losing everybody. My thoughts created this universe. Can they get me out of it again?
Quaice was the instigator of these thoughts. He was tied to this idea of people disappearing at a very fundamental level in Beverley's mind. So he is the first to vanish, and people continue to vanish at a regular rate even though they are still physically within the bubble's boundary.
