Saying Conan doesn't reveal his identity is wrong, there are 10 people who know about Conan true identity, and Conan told 2 of them. (Link contain spoiler) Conan never told Ran because it's dangerous for her to know the truth. Professor Agasa warn him about it at the first time he know about Conan, Second Chapter - The Shrunken Detective
Knowing her personality, Ran will risk her life to save others and most likely too help Conan to fight the Black Organization. This chapter (chapter 434 Rotten Apple) is when Ran tried to save Ai from Vermouth, she risk her own life to save her
On several occasion, Shinichi wants to tell Ran the truth about his identity but chooses her safety above their happiness.
Beside, different from the others who know Conan's secret, Ran might unintentionally tell anyone about it. Closest possible, she will tell her best friend, Sonoko. For example, when Ran accidentally tell Sonoko about Oogami. (He's a wrestler who cover his real identity by wearing a mask - Chapter 389 The Man Who Can Never Be a Wolf)
At one point Ran knew that Conan is Shinichi and actually waiting for him to tell her by himself, but as always Conan was able to trick her again.