I've never worked for any of those brands, so I don't have first-hand experience. (I worked for a corporate-owned/operated Radisson, an independent luxury hotel, and at Disney World.)

My guess is that it's corporate culture issue, not an operational one. Giving late afternoon checkouts can potentially cost money because housekeeping needs to stay open later to clean the rooms. As most hotels are franchises, owners are reluctant to spend extra money on a guest that might never return to their hotel.

If the Hilton Corporate Office and/or Regional Sales Managers don't view guaranteed or extended late checkouts as important, it's not surprising that franchises don't go out of their way to offer it. I never saw any data on the costs/benefits of offering free late checkouts. But, since other brands market it as a benefit, I'd bet it has some positive benefit to the brand.

Anyway, that's my take on it. I'm also glad that some people appreciate these types of posts. I sometimes am reluctant to share some the "insider" stuff I know because I don't want to come off sounding like I know it all.