My friend who is studying in the States told me he has been rewatching old episodes of Malcolm in the Middle on Netflix, so I thought I would too, but they are not there. They are in the US region only. So I wondered why different countries have different content like this? Is it a licencing thing?

Thanks

Yep. You just answered your own question. If someone else has exclusive rights to a show in this Country for example it is not going to be available on UK Netflix. There are plenty of ways you can access other regions of Netflix all explained in other threads on this forum.