Irregular/Atypical Deck Joist Spacing
Because that works out with other common deck and building components and aligns with decades-old convention. For example:


Decking lengths are usually in 2' increments
Decking is built for specific spans with respect to strength and stiffness
Floor sheathing is 8' long (which sets the standard)
Joist bridging is sized for 16" centers
Insulation is sized for 16" centers (when you convert your deck to a three-season porch)


Etc., etc. Incidentally, 19.2" is also a common joist interval because it works out to 8'. 

In your case there's probably no immediate reason you couldn't do something non-standard, if your decking is rated for the span you go with. Code probably doesn't address joist interval as long as it meets the requirements of the other system components.