Maybe they discovered that it wasn't satisfying to use in the beta client, or that they just don't like counter-play / fast-paced decision making.
I have no idea why it isn't capable of being interrupted, but lets hear some ideas about why people think that this is the case.
It's because Galio slows himself down while channeling, can only hold it for so long, and in order to make the most of it practically has to be in the middle of the enemy team. CCing Galio already will make it easier to avoid his W by keeping him from getting into position; making it cancel the ability itself would make it feel that much worse and make it laughably easy to avoid.
Because he sacrifices speed and every mage and their gandma has CC in their kit.
He can still be stunned, just not have his W interrupted.
As a heavy Galio player, I can tell you it's because the flash+ult combo isn't as perfect as it should be. Galio ult can be interrupted by hard CC, and often is if someone's nearby and out of range. With the CC protection, it's going to be much more consistent.
Because he slows himself hard to do it.
