It's not uncommon. It's easier to collect than it is to classify. Eg, here's a news report from 1988 about how "[a] 70-foot dinosaur known as Happy stood prominently on display at the Cleveland Museum of Natural History for nearly 25 years before researchers discovered it was a new species." http://articles.latimes.com/1988-09-25/local/me-3747_1_natur...

It points out that "Museum storerooms have become rich hunting grounds for new species because a lack of money and staff has delayed the identification of bones excavated up to 60 years ago" and that "75% of the museum's collection has not been examined closely."

Nor is it a recent issue. The HMS Challenger expedition of 1872–76, which was the first real oceanographic expedition, took another 19 years to publish everything. (See http://en.wikipedia.org/wiki/Challenger_expedition#Findings_... ).

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+.