They set the system-wide defaults at whim. I don't know how much server resources it would take to set more than a million individual defaults.
I have no use for a pedigree view of my trees' root persons, the latest 'default'. Until they installed the non-improvement latest pedigree and "family" views' versions, the default was the pedigree view of last person viewed -- which also is useless for me. These views only instigate useless tree hints that I've already ignored. I have no idea whether any user/customer requested the new default.
This is why I go to a tree from my browser bookmark to the tree overview page from which I can easily see a link to the last person viewed, or select where to start.
Setting a browser bookmark bypasses the page-load time of any Ancestry.com page, and has the advantage of working when non-Tree parts of the system are down. For me it is a one-click, no-loading-wait-time simple option.
Even if you were able to set a personal preference on the site for default opening page, you could not set that preference for others (who might not have chosen that preference themselves anyway).
If your tree invitees log in and then click on the link you sent them, the link should send them directly to the linked page. Are you saying the link does not work for them? Are you sure they are clicking on the link from your message?