why all the dimension creating are banned

Why? Performance issues, lags, low compability for servers, crashing server - more cons than pros

 

LordEreh actually explained all the reasons why it isn't allowed. In a larger community server it would be a tremendous strain on resources to allow things like mystcraft dimensions etc.

 

I am unsure if this issue still exists in 1.10, but 1.7.10 has horrid WorldGen.
1 single player generating new chunks could timeout everyone on the server, or even crash the server.

We always pre-generate all chunks before we let anyone on any of our new servers, because it is literally unplayable otherwise.

This is in parts of (back when 1.7.10 came out) they re-did some worldgen, as well as some mod-developers not having even the most basic understanding of how worldgen should be coded.

At least the Forge community has started cracking down on Mod-Developers adding in WorldGen causing Chunk-cascades* in recent versions of Minecraft.

I would like to finalize this as such: For 1.7.10, it would be impossible to implement in a functionable manner.
1.10+? Perhaps. It would depend on the modpacks, and the amount of WorldGen inside of them. However, IF this would turn out to be do-able, it would have to sit on the back-burner for some time. The current focus right now is on the Monetization issues MyM is facing.

*Chunk-Cascades: The act of loading one chunk during worldgen (first login for example) and that chunk contains something that loads another chunk (a tree that crosses chunk borders etc), and this keeps happening... Over and over and over again. Like this.

 

Now I know why my 6 high-density worldgen mod and 100 other mod modpack was crashing the game upon creating a world...

@toppic
Nobody mentioned storage space.
So I'm mentioning storage space.

 

Long story short if you want a playable server you need these options disabled, if you want an unplayable nightmare then they should be enabled.