Because that's still not going to solve the problem.
FCs get special attention because there is FC content locked behind ownership of a house. SE needs to make certain that there are available plots for newly formed FCs.
Mixed wards were the standard prior to the introduction of the lottery last year. It wasn't a subject of controversy until the purchase timer was added and FCs became the method of reselling houses to first time buyers then later the ability to resell at all when relocation stopped bypassing the purchase timer.
SE brought back the mixed purchase wards because having a mix of FC and personal houses together is theoretically better for creating active neighborhoods, and because FC demand had reduced enough that they wanted personal buyers to have a chance at more houses without jeopardizing the ability of FCs to get mediums and larges. If there's a medium available in a mixed ward but none in the FC only wards, then a FC trying to get a medium is going to go after what's available to them.
I would be very surprised if FCs are going after smalls in the mixed wards unless they have members with personal houses already in the same ward.