Why old KDE LTS 5.18 on Leap 15.3

Why is OpenSUSE 15.3 sticking with KDE Plasma 5.18 LTS?

It is the same version that came with 15.2
Would be better if they had used Plasma 5.21
Do we have to wait until Plasma 6 LTS before OpenSUSE Leap upgrades KDE?

That’s how Leap rolls… you would need to wait for Leap 16 to appear…

Leap uses KDE Plasma LTS. You may install newer one.

So the reason is, they want to stick to Plasma LTS, and 5.18 is still the latest LTS.

IMHO yes. And it works.
You may install newer KDE.