Why prokaryotes don't have nucleus??

Question

is nucleoid creating some specific benefits to them??

if yes, then how?

Answer 1

Prokaryotes don't have a nucleus because they retained the ancestral trait of not having a nucleus.  They did quite well without one and currently have cellular functions that rely on not having a nucleus.  For example, bacterial cells have evolved some gene regulatory mechanisms, like attenuation, that require transcription and translation to be coupled.  If they put their DNA in a nucleus, then this gene regulation that works quite well for them now, would not be able to function.  

So, they are managing quite well without a nucleus and evolving one would then impact their current cell functions.