The rogue preview confirms my suspicion that the change was made because 4th edition has an increased number of smaller movements. In the 3.5 system, where diagonals cost first 1 then 2 then 1 then 2, you couldn't move 10 feet on a diagonal. That was okay, since there weren't that many situations where you would measure 10 feet. In one of the few common situations where that mattered, 10 foot reach, the designers introduced a "hack" that let a 10 foot reach cover two diagonals. The awkwardness did extend to a few other things, like 10-foot radius spells. Notably, you could be directly adjacent to a person with a spell like "circle of protection from evil" and not be affected (see attached diagram).
In 4th edition, it seems a number of things will move 10 feet. We've seen Deft Strike, plus potentially Positioning Strike and Tumble. Plus there's other ways, like using two move actions to shift in a turn, where you would end up moving 2 squares. Noticeably these are measured in squares, so the idea that you couldn't move 2 squares diagonally since that's actually 3 squares is a little silly.
I certainly see that the 3.5 system is more accurate over longer distances, but when you're only measuring 2 squares of distance, both systems are exactly as accurate, with the weird complication that you just can't measure a 2-square distance diagonally in the 1

1:2 system. So, if the idea is to make combat more dynamic by letting characters move shorter distances more often, the 1

1 system works better, even though it is notably inaccurate over long diagonal distances.