Why has Airbus put an altitude limit to 21000 on having gear down

[TSHEKUDU]

Hi there to all the brain boxes of the world.

I have a few questions.
1- why are we limited to 21000 feet with the gear extended?
2- does having a gear door or doors not closed limit you to 21000 feet (I understand the fact that you are limited to a speed of 250/.55 because you might blow the doors away) but can you climb to a higher altitude?

A bit of back ground, I am on the A380. The only info I can get from the FCOM states, that I cannot operate and have my gear down at higher altitude than 21000. Is that valid for a gear door or doors?

[KBPsen]

In the old days they used to have both IAS and Mach limits in the FCOM but no altitude limits. These days you just have an IAS and altitude limit.

You will likely find that 250 equals .55 at FL210.