I think the question was why an imperfective verb (and not a perfective one) was used.

I do not know an exact rule for that, but I'll try to explain how I feel it.

I thought over that, and I think we use imperfective verbs when we want to express a general experience in the past.

So, "Мы ездили в Париж" - it's a kind of "general experience" meaning roughly "We have an experience of visiting Paris".

That is not limited by the verbs of motion. Any other verb can be used like that:

- Ты знаешь, это интересная книга! - You know, this is an interesting book.

- Да, я знаю, я читал её. - Yes, I know, I read it. (I have an experience of reading the book).


I believe grammar books might provide some better explanation, but that is what first comes to my mind.