I remembered growing up and watching Power Rangers not really understanding why cast members left, I even had my hopes Amy Jo Johnson would return on the set. In Zeo, I thought she would return on the set but sigh, that STUPID Dear John Letter which was very OOC of Kimberly to do happened. To be honest, that incident led me to write that rather disturbing even that her character got raped behind the scenes in Super Sentai vs. Power Rangers by Radiguet my big bad who's now lusting after ladies. So here's been my theories surrounding the Dear John letter:
1.) Amy Jo Johnson was considered the "eye candy" of the series but as her contract had expired, well yeah... so rather than renew it she chose to leave the set so she could explore other opportunities beyond Power Rangers which I think career-wise is a good decision. So I guess the writers with Zeo were trying to get even with her or what by making her character look bad off-screen.
2.) The arrival of Kat Hillard wasn't handled properly. Just because she replaced Amy Jo Johnson doesn't mean her character should become Tommy's new girlfriend. For me, it was perhaps they were afraid of killing Kimberly so they did the stupid alternative of the Dear John letter. Pretty much I felt like the Tom/Kat pairing was really that forced.