The reason South Park has musical numbers and Japanese influences is because one of the creators double majored in them!

Trey Parker was born in Conifer, Colorado, on October 19, 1969 to Randy Parker, a geologist, and Sharon Parker, an insurance broke. He also has an older sister named Shelley(Stan´s family anyone?).

He met Matt Stone (co-creator of "South Park" (1997) while attending the University of Colorado in Boulder, where he had a double major of music and Japanese. This explains the well done musical numbers and Japanese satire in the show.

While at UCB he wrote, directed and starred in Alfred Packer: The Musical (1993) (aka "Cannibal: The Musical!") Based on a true episode in Colorado's history.

After graduation from UCB, he and Stone were asked by then-FoxLab executive Brian Graden to create an animated Christmas card for his friends and family. The now infamous short, titled The Spirit of Christmas (1995), led to "South Park" (1997).

He also voices Stan Marsh and Randy Marsh in the series!

(Source)