2 possible reason I could think of:
1) Marketing, its been awhile since the Volume 6 and a Hellboy in Hell is an easier sell then Hellboy 7
2) In Hell is supposed to be the "final" Hellboy story and the Hellboy Library editions are kind of a chronological reading order and so actually reprint the trades in a slightly different order
http://hellboy.wikia.com/wiki/Hellboy_Library_Editions if we get anymore Hellboy stories they should come before Hellboy in Hell and so would fill a theoretical Volume 7 (8, 9 etc)
Honestly its probably the first one but I can always hope for the second.