That and It
In the English language when we use the word "that" you are usually referring to something or someone that is not located close to you. So for example if your dog , ball or friend is across the street , you say " That is my dog.", " That is my ball.", "That is my friend." .
In contrast the word "It" is a singular personal pronoun. You normally use "it" to refer to objects (objects you can or can't touch) , non- living things and to highlight something that was said earlier.
For example . The building is colourful . "It"is beautiful.
So let us examine the sentences .
Sentence 1
"We had a conversation for 30 minutes. *That* was about our simple profiles, cultural shocks through traveling, religions and so on. "
Using "that" would show you are referring to the conversation as being a person or an tangible object not being close to you , but some distance away from you . Hence it is grammatically wrong.
Sentence 2
We had a conversation for 30 minutes. *It* was about our simple profiles, cultural shocks through traveling, religions and so on.
Using "It" shows you that you are highlighting something stated from the 30 minute conversation earlier. So using "it" points out that the conversation was about simple profiles, cultural shocks through traveling, religions and so on.
Hope this helps