Visitor Visa - B2

Dear All, i am being invited by xyz Institute to attend their exam assessment conference? The purpose is to asses the exam and give feedback about it. Why i am being invited is that i am their active member and posses the charter. They invite people from other countries so they can have diversity of opinion as they are global exam and somehow not only related to US. They are trying to improve their image worldwide.

I got the invitation letter from them stating all the reason for attending conference.

My profile: More than 8 years of travelling experience
Been to Uk, Turkey, Srilanka, Dubai, Saudi, Maldives couple of times
Director Business planning and strategy
Monthly Salary around 350k
Married and 2 kids ages 5 and 3

First interview:
VO: Hello
Me: How are you
VO: Show me your passport
Me: Handed over
VO: Purpose
Me: To attend exam conference
VO: Why you giving exam?
ME: I am going to evaluate it
VO: Whats your relationship
ME: I am their active charterholder and member .. Also i am selected based on my skills
VO: I dont understand why would they invite someone all the way from Pakistan to evalute US exam
ME: Give more reasons of my relationship
VO: Have you travelled to other countries
ME: Yes i mentioned countries
VO: Show me old passport
ME: Showed to her which has 10 year uk visa valid
VO: Not sure why would they invite someone all the way
ME: Didnt say much

She paused and said sorry i cant issue you visa
After interview i thought that she didnt understand the reason of conference and confused about my selection. I could have done better with more explanation so i should appear again. A

Second interview:
Greeting exchanged
VO: You appear 5 days back for interview as well
Me: Yes
VO: What has changed from last week?
ME: To be honest nothing changed but i thought i could have explain things better of my visit and also i have strong ties with my country
VO: Strong ties is not an issue
VO: Why you go for exam?
ME: I am not going to give but to evalute
VO: Dont understand why would they invite you for us exam
ME; its a global exam and i have the expertise therefore i am being invited. You can sponsor letter
VO: No need for letter
VO: Sorry i cant change the decision
ME: Let me explain in detail
VO: Soorry gentleman there is no need
ME: Its important for my career and i can explain in details
VO: Sorry, you can leave

I dont understand why they have rejected my visa when i have great travelling history, strong home ties and also solid reason for conference? Why are they curious that i will evalute us based exam. i told them all the reason

Anyone with feedback