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
:Kingdra:,
:Ludicolo:,
:seismitoad: and,
:Barraskewda:, Hydration
:Manaphy: with Rest, and Choice Specs
:Keldeo: all come to mind when it comes to VERY strong offensive rain abusers. Rain Dish
:tentacruel: can serve as a stall asset with black sludge, and
:Blastoise: can run both a sweeper Shell Smash set or a defensive Rain Dish set with leftovers and Protect. Rain setters like
:Pelipper: and
:Politoed: 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.
:Pelipper: 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
:gigalith:,
:Tyranitar:, and
:Rhyperior: benefit greatly from the SpDef boost sand gives them, as they have already high Defense stats. This also applies to
:terrakion:, which as ok bulk, but the boost also helps. Sand Rush
:Excadrill: and
:lycanroc: become unstopabble after a Swords Dance boost in sand.
:Hippowdon: serves as a great sand setter and physical wall, and
:Tyranitar: 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