Barclay was probably that awkward kid everybody picked on and it's left him with low self esteem issues. Wouldn't surprise me at ALL if he had to deal with a ton of bullying most of his life and career. Maybe he never spoke up because either nobody bothered to do anything about it or he was too afraid to make waves and make things worse for himself.
I may or may not be speaking from personal experience. >>;;
Yeah. It was a bit sad to see that in the enlightened 24th century, there was still people who got made fun of and excluded.
Picard and Guinan seemed fair to him. And he did get accepted later on. Deanna coming to check on him when he reappeared in Voyager.