The upcoming section has many submissions with hundreds of votes. Why haven't they been promoted to "Popular"?

There is no guarantee or any hard numbers of votes to promote a
submission. In a nutshell, diversity is the key to getting a submission promoted to “Popular”, the Gnommie homepage. Gnommie uses a sophisticated algorithm that takes into account the number of votes, views, shares, comments and several other factors to determine its relative ranking. Until the algorithm receives the diversity it needs from all the factors, the submission will remain in the upcoming section.