Why is sandmon a no but sunmon a yes? Because the pokemon that a sandmon would boost are /already good in OU./ The difference between sun and sand is that sand doesn't really have ABUSERS—in an ideal world, your best possible sand team would still probably be shit like Ferrothorn, Landorus, Jellicent, Tyranitar...
In a sun team, there are absolutely hordes of Pokemon that get a huge boost in Sun that don't see usage because they are either a) too dependent on weather and not strong enough in rain (Darmanitan, Victini) b) impossible to run when you're already devoting so many teamslots to a decent backbone (Ninetales, Dugtrio, Forretress is basically half of every sun team already.) By making a Pokemon that really makes sun the dominant weather, you're increasing the viability of a shitton of UUs that will now be good in OU thanks to not having to worry about rain beating them. If you make a Pokemon to boost sand, your team will be: Tyranitar/Keldeo/Landorus/Ferrothorn/Scizor/CAP5.
tl;dr: a sunmon accomplishes both boosting underused types AND hindering overused types, whereas a sandmon only does the latter.