Dawn was designed to be a female take on Ash for little 8 year old girls. Some kids grew up watching DP as their first saga, and that's who the writers aimed Dawn at...not older fans. She basically had a similar type of personality as Ash did, just with a bunch of girliness and a love for fashion.
The other girls were designed to have more of a dynamic among the main cast and their own storylines. The writers obviously wanted to try something different with Dawn, but after that they went back to the usual group dynamic routine as seen with Ash/Iris/Cilan.