The Natural History museum in the UK has shelves full of samples dating back >100 years that have never been looked at. Furthermore, even ones that have been st

The Natural History museum in the UK has shelves full of samples dating back >100 years that have never been looked at. Furthermore, even ones that have been studied can be revised in the light of new evidence. So primarily it's a question of man power and triaging samples. Secondly you need a lot of evidence before declaring a new species and various formal steps should be taken (providing a reference sample etc), so that process in itself can take years+.