Why is there no Rashi on Yerushalmi? What can I learn instead?
Is there anything that can replace it, and why was it not written?
Double AA covered the main commentaries on Yerushalmi. Here are a few very useful contemporary ones:
Lev Yerushalayim (on all of Yerushalmi, I think)
Commentary of Rav Chaim Kanievsky (example here)
The Artscroll Yerushalmi (in progress)
A common commentary to the Yerushalmi that serves a similar function to Rashi is the Penei Moshe written by Rabbi Moshe Margolis. A volume from the Talmud Yerushalmi with his commentary can be seen here.
Other commentaries that follow a similar pattern are Korban Ha'edah by Rabbi David Frankel and Chiddushei Ridvaz by Rabbi Yaakov David Willowsky. These are all available in many printed Yerushamis today.
As to why Rashi didn't compose a commentary on the Yerushalmi: he might have if he had ever finished the Bavli. Unfortunately, he died before doing so in the middle of writing his commentary on Bava Batra (he made it until page 29a).