If I may be blunt… this doesn’t look professional at all. There are a few nice parts, but overall it looks very plain and cheap. Plus, spacing is erratic and messy, many elements are misaligned, etc. This is far below the level of detail and attention to get a theme approved here, especially in a category like forums.
It’s hard to give any specific advice or reason beyond that. If the reviewers like your future designs, they’ll let you know about any specific issues that need addressing, but otherwise you will not get any feedback with your rejections.
I’d suggest looking through some themes that have already been approved to get an idea on what the reviewers are accepting: https://themeforest.net/category/forums