They are recommended due to their stability, package management system, availability of packages and the fact that ISPConfig is developed on Debian.
The performance of different Linux distributions is similar, so this is no point for selecting a specific distribution. Regarding space usage, Ubuntu and debian are smaller then the default installs of e.g. suse or Fedora. But of course you can strip other distributions down to the size of a debian system manually.
The most important points are the stability and availability of packages. Debian and Ubuntu have all required packages in their main repository, this means that all packages fit togehter. If you compare that with e.g. Centos, then you will see that Centos has only a few packages in its main repository, all other packages have to be loaded from third party repositorys which cause sooner or later package depenency conflicts e.g. when a packacke from repo a requires a library in version 1.2.3 while another package from repo b requires the same libary but in version 1.3.4. These problems will not occur with Debian and Ubuntu.
We migrated most our customers servers to Debain and the systems are running much smoother now and the reuired maintenace time is much lower.