I can't really answer "why" Marriott and other programs limited extended stay property earnings to 1/2 the rate of the other brands. But I've always just "assumed" it was because as an extended stay property, once someone has chosen where they will be staying (for an extended period), Marriott doesn't have as much a need to "entice" them to keep staying there. Most people will not want to move from one apartment to another just for some points.

Also, I thought Starwood limited the length of stay to something like 60 or 90 days where points could be earned? If someone's stay lasted longer than that, they wouldn't get points for that stay. Marriott has never had length of stay limits like that. I've known people who've stayed more than a year in an RI and got all the points from it (with not having to check-out for a night or 2 and then check back in anywhere in there.) So maybe both had their "limits" (one was number of points earned, the other the number of nights for a stay). And guessing that the original premise for extended stay hotels was, well, for extended stays, then on actual real extended stays, Marriott's approach might provide more points for folks than the old Starwood approach. (Unless someone played the check-in/out for a night or 2 during the stay, which most non-FTers probably wouldn't do.)

And I'm guessing that most FTers probably don't fit the true "extended" stay mold, and as such, we aren't necessarily the target audience. Thus it might make sense for most of us to see other, non-extended stay brands, as more appealing for earning points.