I suspect it has to do w finance; i.e. the ROES system is ordering / billing / etc. It appears to be totally automated which may explain their cost structure. In addition it was probably too difficult to integrate w Zenfolio & finally, how do they share profits.
WHCC is pretty well known & may not feel the additional exposure they might get from Zenfolio users is worth the effort. All in all it is a shame as it is such a good, well-rounded service.
I don't know about you but mugs, aprons, mouse pads & the like is not of interest to me. I imagine it might be of interest to compainies @ xmas time, etc. -- high volume.
I chose Zenfolio because it seemed more elegant than Smug Mug & some others which are nice services but too cutsie for me. Just my .02c U.S., currently worth about .01c around the world.