I don’t truly want to speculate why this is, but observe the typical cartoon ensemble: at random I’ll choose Scooby-Doo (Hanna-Barbera), the Simpsons (Groening), and Looney Toons (Warner Bros). I’ll list the actors in each, and name which characters they’re famous for.
Scooby-Doo
Don Messick: Scooby-Doo, Astro, Papa Smurf, Hamton J. Pig, Boo Boo, Bamm-Bamm, Muttley, et al.
Frank Welker: Freddy Jones, Dynomutt, Mxyzptlk, Iceman, Megatron, Ray Stantz, Slimer, et al.
Casey Kasem: Shaggy, Robin.
Nicole Jaffe: Velma.
Stephanianna Christopherson: Daphne.
The Simpsons
Dan Castellaneta: Homer, Grandpa, Mayor Quimby, Pimply Teen, Krusty, Groundkeeper Willie, Barney, et al.
Harry Shearer: Burns, Smithers, Skinner, Lovejoy, Hibbard, Otto, et al.
Hank Azaria: Moe, Wiggum, Snake, Apu, Frink, et al.
Julie Kavner: Marge, Patty, Selma.
Yeardley Smith: Lisa.
Nancy Cartwright: Bart, Ralph, Nelson, Chuckie Finster.
Pamela Hayden: Milhouse.
Tress MacNeille:Agnes Skinner, Daisy Duck, Babs Bunny, et al.
Looney Tunes
Mel Blanc: Bugs, Daffy, Elmer, Yosemite Sam, Foghorn Leghorn, Porky Pig, etc., etc., etc., Barney Rubble, Dino, Speed Buggy, Mr. Spacely, et al.
June Foray: Granny, Witch Hazel, Betty Rubble, Rocky J. Flying Squirrel, et al.
For every June Foray or Tress MacNeille or Christine Cavanaugh (Dexter), there’s two or three men with equal or greater voice versatility: Welker, Blanc, Messick, Jeff Bennett (Johnny Bravo), Azaria, Castellaneta, Shearer, et al.
One is tempted to draw a few conclusions from that: either women get fewer opportunities (fewer roles written) that showcase their vocal versatility, or women aren’t encouraged to be as good at it (which seems odd, since there’s no shortage of actresses) or casting directors cast men and women differently (men for versatility, women for single characters) or some other reason. There just don’t seem to be as many versatile working female voice artists. The most successful ones have little versatility, and the most versatile ones aren’t famous outside the industry.