So why is AVA not available in all seats?

According to the CBA 25N4

4. Advance Volunteer Assignment (AVA)
The Company may advance assign a VLT trip prior to the open time assignment window subject to the following:

a. AVA shall be available and assigned to all eligible pilots beginning 7 days prior to showtime for that assignment, based on the reserve forecast.

b. A trip is eligible for AVA assignment only if the trip has a showtime:

i. between Thanksgiving day and December 31; or

ii. during a bid period in which the Company has requested voluntary vacation cancellations in the pilot’s crew position; or

iii. during a bid period in which the Company has involuntarily canceled a vacation in the pilot’s crew position.