why did billy leave
 

why did they let rockey and billy leave and let justin become the blue ranger

David Yost ( Billy) had a falling out with the production staff. It had to do with money issues and being unhappy with the direction his character took during Zeo. Yost actually walked off the set. Which is why they had Billy rapidly age in the Zeo/Alien Ranger team. When Billy's age was restored his face was blurred in the message he sent back to Aquitar. This was because they were using a stand in for Yost. Since Yost wasn't avilable to shoot that scene, being that he walked off the set and all.

I am not sure why Steve Cardensas ( Rocky) left. But it seems like he left on good terms. I guess he just wanted to move on. I think he started a martial arts school. I am not sure if that had anything to do with him leaving. They wanted him to make a cameo in "Forever Red", but they couldn't get in contact with him. Cardensas recently appeared at Morphicon ( the recent Power Rangers Fan Convention in LA)

Blake Foster (Justin) was brought into the show to appeal to the kids. Since Shulky Levy ( one time Executive Producer) thought a kid Ranger would appeal to the kids. This was due to Big Bad Beetleborg's ratings success. Which of course featured young kids as Superheros. The Justin experiment didn't work however. Blake ended up leaving the show after Turbo wrapped Production.

about blake, i heard anywhere long time they say reason why he leave because
of blake's mother worry about his safe and too young that all i read about it.

They didn't live in Communist China. Billy was no longer needed on Earth. He was dying. He could be saved on Aquitar. The others didn't want to see their friend die, so they let him go.

Rocky could no longer do the job, even in Communist China they would have let him leave. Justin was made Blue as payment to keep his trap shut about what he overheard.

In the real world both actors wanted out.

Actually from what i read from an article, they wanted to match it from various Super Sentai, as many of the rangers in the past were kids. The Japanese white ranger from MMPR season 2 was a 8-9 year old, and OhGold (Zeo Gold) was a kid too.

This post just reminds me, my high school english teacher used to date "billy" the blue ranger. In middle and even high school myself and all my peers still watched PR... until our teacher told us that. And we realized, damn, billy's OLD! lol! Plus, everytime we saw the show we thought of HER and no one liked her, she was a mean teacher!