Your portfolio doesn't show any examples of what you want to sell.
This is probably the #1 reason Creators are rejected, if you want to sell characters make sure you portfolio link has characters in it.
You did not adhere to the market creator guidelines.
You can find the general product guidelines here, make sure you have followed all of them.
Your submission doesn't meet quality standards.
Some of the most common things we look for as far as quality are great topology, modeling, materials, lighting, animation, composition, etc. A great way to find out what type of quality we are looking for in our products is to look at other products on the market.
Your product does not hit a wide enough market.
Products need to be commercially viable, basically, they need to offer value to a wide range of people. Take a look at the products already for sale on the market for better examples.
Your submission is too similar to existing items.
Though there may not be anything wrong with your file, we feel that it is to similar to products we already offer on the market or there is not enough demand for this type of product.
You have resubmitted a previously rejected product without significant changes.
We don't feel you have made enough significant changes before resubmitting it again. Repeated attempts may result in a temporary or permanent suspension of your account.