The "good" ones:
- have more contrast, the darkest and lightest shades of each colour are more distinct than in yours
- use hue-shifting, the darker shades of each colour aren't simply darker, they're also redder/purpler
- have shading, parts of the characters that would get less light (like right under the "skirt" on the second one, and under the chins) are darker
- have consistent lighting. It's from the front, which makes it look much flatter than it otherwise could be, but it's better than nothing.
- the details aren't just random shapes, they represent real things, like shadows under pecs, armour segments, etc. Yours, on the other have has a repeating pattern that doesn't look like anything. The "good ones" also have fainter shadows that create additional smaller details that yours lacks since you're only using one shadow colour.

I recommend studying different sprites, however. These have an overload of detail, static poses, and a general lack of clarity. It's much easier to study better work, as you'll pick up fewer bad habits, and probably have an easier time seeing why the artist made the decisions they did. Learn to convey form first, then worry about detail - detail should never overwhelm the form, or you'll lose clarity.