MegaBanette69, I'm not completely sure, but it's probably due to the excelent mons that can abuse both rain and sand.
Rain: Swift Swimmers like
,
,
and,
, Hydration
with Rest, and Choice Specs
all come to mind when it comes to VERY strong offensive rain abusers. Rain Dish
can serve as a stall asset with black sludge, and
can run both a sweeper Shell Smash set or a defensive Rain Dish set with leftovers and Protect. Rain setters like
and
are also very good Pokemon and don't serve a purely weather setting role, as they can run not only the Damp Rock, but also items like Choice Specs.
can also run Heavy-Duty Boots and Defog, a very useful role compression along with it's good base 100 Defense, serving as a defensive wall, weather setter and defogger.
Sand: Sand abusers like
,
, and
benefit greatly from the SpDef boost sand gives them, as they have already high Defense stats. This also applies to
, which as ok bulk, but the boost also helps. Sand Rush
and
become unstopabble after a Swords Dance boost in sand.
serves as a great sand setter and physical wall, and
can be defensive, offensive, supportive, and much more. Sand also chips at the opposing team, along with most rock/ground mons getting Spikes and Stealth Rock.
TLDR: these two weathers are very flexible and have very good abusers