The difficulty arises because there is a big difference between what the ODP is and what people perceive it as being or would like it to be. The two different perceptions does lead to frustrations on both sides.
From the outside. people see a large directory of website listings and believe it must be some sort of corporate entity who are geared towards listing sites. In fact it is a group of people following a hobby and listing sites is only part of the work involved. So, for most people who react to the ODP as a result of their perceptions, a better understanding of the ODP actually is, is what is needed.
The ODP is about developing categories and the suggestion pool is only one source of websites. I list most sites that are suggested but most of the sites that I list are not suggested. I regard a suggestor as someone who is helping me develop a particular category by suggesting a site thus saving me the effort of finding it when I get round to that category.
So the answer as to why any particular site is still unreviewed in a category I could edit, is that I can only devote so much time to editting and this varies. Sometimes I am enthusiastic and get lots done, other times I feel a bit jaded and do other things. Sometimes I feel a particular areas needs my attention and as a result I ignore other areas. The same goes for all the other editors who could review that category.
The answer as to why any particular site is still unreviewed in any category is that the number of editors that could edit any category is only a few hundred whilst the number of categories is approaching 600,000. This means that were each editor to go through systematically and edit in a different category each day, it would take at some 3 years to cover the lot. But a 'day' of editing means different things to each volunteer; from 8 hours because it's Saturday, raining and nothing else to do to 0 hours because it's my night to go down the pub, over-indulge and get carted home in a wheelbarrow. And of course in that time they may or may not be able to carry out all the work needed for that category.
Many people look at their site to find some reason why it is not be listed - is the editor a competitor, misogynist, racist, of the opposite political persuasion, jealous, an alien being intent on overthrowing the earths economy etc. etc.
If your site is listable (which at this stage only you know - see guidelines), the reason it is not listed is that no one has reviewed it yet.
Such a simple answer is however often less than satisfying
regards