It means 'I couldn't get it' in the sense of 'I couldn't obtain it'.

But why is it "couldn't", not "didn't"?
I believe "couldn't" is 'irenakatta', and 'iranakatta' is "didn't".

My skills in both English and Japanese aren't any good, so I would be happy if someone explain this to me.

You're right. 入らなかった is a plain past form, it's not a potential form as it should be.
I have doubts about the sentence in general though. I'm by no means experienced enough with japanese but I've never seen this expression.

It actually means '(It) did not get into (my) hands', the subject being the thing that one failed to obtain. One usually expresses oneself like that. Of course it is not exactly literal.

In order to say 'I could not' one would have to rephrase it like '(watasi ha sore wo) te ni irerarenakatta' or '(watasi ha sore wo) te ni ireru koto ga dekinakatta', using 'ireru' instead of 'hairu', as you suggested, but since the English sentence is so short and colloquial, I wonder if it isn't good the way it is now.

Anyway, I would ask bunbuku, mookeee or someone else to comment on this as well.

This expression is commonly used when you tried to get something but couldn't.

I think the English matches the Japanese.