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