Because of his colors. Think about it; complementary colors (blue mane, orange body) make him look a bit too 'showy.'
For comparison, Twilight Sparkle is, for the most part, monochromatic. Her mane is dark purple, her mane highlights are magenta and violet, and her body is a light purple. Monochromatic can convey the sense of calmness and elegance.
As for Flash's complementary colors, they show contrast and create attention towards him.