Well, this was a LONG time ago. I am not sure anyone who was part of that decision is even around anymore, so it's hard to answer that. But, from what I remember of it, the global block skill was created in order to make it a bit easier to kill someone, for everyone, because killing people had grown increasingly harder.
I am not sure when you started playing, but unless you did not know, in the 90s LAG (that is, Internet latency) was a big part of the game, especially in PVP. The LAG was due to the poor Internet speeds of the time, but it also served to make PVP way more dangerous than it is today, because due to the LAG it was very hard to run away from a fight. So, dying in PVP was a big part of the game. When Internet speeds increased, and LAG more or less disappeared from the game it became way harder to kill someone since it was so much easier to quickly run away. Unless you were a knight of course. I was a knight for most of my days as a mortal, and yes, I think it became way too powerful and too unfair. LAG had kind of evened the playing field a bit, but without LAG knights had an even bigger benefit in PVP.
So, global block was introduced, but Knights still had an edge, though, since they could train the skill higher than the general population (I don't know if that is still the case). But the old and new block is almost two different abilities. The old block could keep someone in a room forever, the new isn't nearly as lethal. So yes, knights more or less lost one of their unique abilities, and you could argue it ruined part of the flavor with the guild, the old block was a defining factor that made knights very dangerous. But, even though I was a knight at the time, and even though it "nerfed" me, I think it was a good and needed change.
Now, again, this is probably almost two decades ago. Knights has undergone at least one big recode since then, and I think they have plenty of unique flavor-stuff that suits the guild very well today. And honestly, theme-wise I don't mind that knights are no longer the goodie side's sure-death murderers