I don't think it's that uncommon
Look at how many BEST OF albums there are in the Oricon 2008 top 100. For example:
EXILE LOVE
EXILE CATCHY BEST
EXILE BALLAD BEST
EXILE ENTERTAINMENT BEST
Amuro Namie - BEST FICTION
B'z The Best "Ultra Pleasure"
B'z The Best "Ultra Treasure"
Hamasaki Ayumi - A COMPLETE ~ALL SINGLES~
Shibasaki Kou - Single Best
And that's just a small part, there are way more best of albums to find in that list.
I guess BEST OF's are just a really big deal in Japan