Interesting, R Yochanan seems to be assigning blame for BHM2 destruction twice – once to Bar Kamtza (it is not clear to me whether it is him continuing this sentence from before or stam Gemorah) and then to Rabbi Zekharya ben Avkolas who was too humble to let his chaverim speak.
Why two possible explanations? Both are allegories to certain behaviors in that generation, and it seems to me that both are actually the same – Rabbi Zekharya ben Avkolas shows similar attitude to Kamtza story: people talk past each other and are not able to hear other opinions.