Why did the United States develop the M3 instead of copying the Sten?

Why develop the grease gun instead of copying the Sten?

The explanation offered is that American designers thought they could do better. The result is judged better to an extent, but more complicated.

The follow-up credits American designers with believing they could improve on the Sten. The grease gun is judged to have done so to an extent. That assessment carries a qualification: it was also more complicated. The brief comparison supplies no detailed measure of performance.