There is really no good reason the switch occured.

There is a bad reason of course.

Industry realized that there is not a lot of profit in steel bikes because they last a long time and people loved them. They started making aluminum and then carbon frames that gave no significant or meaningful advantage to the cycling world. Moving away from quality and longevity into a more disposable market which consequently made more profit. That was the end of the road for the 27'(630) wheel.

Sad but true.

On a side note.
If you are into numerology, you'll notice that both 27' or 630 mm make #9.(2+7=9 or 6+3+0=9). Also, If you keep adding 9's you'll always get a 9. For example 9+9=18(1+8=9) 18+18=36(3+6=9) 36+36=72,,,, and it goes on like that forever. This is not possible with any other number. Talk about a perfect circle. The sacred geometry. 360 degrees.