The reason is historical going back to before there was a Nook. As Harry said, PDB is a file format referred to as eReader. In early 2009, B&N acquired Fictionwise, including their ebookstores fictionwise.com and ereader.com, in an attempt to have an online bookstore presence before they launched their own name-branded ebookstore. These websites primarily sold books in the PDB format. See linked article.
http://news.cnet.com/8301-1023_3-10190554-93.html
By the end of 2009, B&N had launched their ebookstore under their own name selling books in the PDB format and now had the Nook e-ink for sale as well. B&N saw that the industry standard for file format is EPUB and that they needed to adopt it. They then decided to convert their ebookstore to selling EPUB. They clearly don't see a future in the PDB format if they chose not to support it on their latest ereader, the NOOKcolor. Here are some links about the conversion to EPUBs.
http://www.digitaltrends.com/gadgets...ns-with-adobe/
http://gigaom.com/mobile/has-barnes-...ormat-to-epub/
In regards to your DRM question, there are many ereaders that read the EPUB file format. For example, I can buy a book from the Sony ebookstore, Borders, etc and read it on my Sony ereader and my NC. However, B&N added a special flavor of DRM to the Adobe EPUBs sold in their bookstore that is not universal. I cannot buy an EPUB from B&N and then read it on my Sony ereader. I can only read it on my NC. There might be some other ereader brands that can read the B&N DRM'ed EPUB's but not as many.
So, in summary, B&N switched their preferred file format from PDB to the more universal EPUB format. They then chose to add a non-universal DRM encryption to their EPUBs.