Basically if any LLU supplier does not think they will get enough of the existing users to cover the costs of installing their own LLU equipment then they won't do it. They will naturally go for the larger exchanges that have tens of thousands of users as the chance of getting enough to cover their costs will be greater.
So either there are not enough existing ADSL subscribers on your exchane yet or it is too small for any LLU supplier to consider at this time. It may also be because the exchange building is so small that there is not actually enough room to put their LLU kit in.