Why does Yasuo shield stay up at max while Mordakaiser goes down to 25%?

If played right, both these champs can gain their maximum shield twice in a fight. Mord can stack his with his W and E from minions and then engage. While fighting, his ult and Q and help to restack his shield to is max value.

Yasuo can engage with his shield up. Then If he then ults, the shield is up again.

So basically, both champs can access their shield twice in a fight if done right. However, there is a major difference in Yasuo defense and mord defense

Yasuo shield, is much easier to acquire twice. He also has windwall which can absorb insane amounts of damage.

While Mord can gain his shield twice, it is much harder. His shield gradually diminish to 25% over a few seconds and he is immobile, making it hard to do the damage needed to acquire his shield. He also does not have a windwall that blocks infinite damage over 4 seconds. He has a heal which is also based om him landing his W to gain that heal. Which again can be challenging because of his low mobility. The W also is just a little bit larger than his AA range so while fighting, targets tend to remain just on the edge of the W ring. This means if they just move a little, they are out of his W range.

Why does Yasuo have a much better shield mechanic while remaining hypermobile? On the other end, Mord has a good shield, but faces huge challenges to fully use it while being immobile.