I thought Eli the black guy who was best friends with Mr. turner, Shawn and Cory. Why didn't they have him not return for future episodes?

I'm going to "borrow" this from the Boy Meets World (https://www.reddit.com/r/boymeetsworld/comments/xmi2vu/theory_about_why_turner_was_written_out_on_a/) sub Reddit.

According to Rider Strong in a Vanity Fair (https://www.vanityfair.com/hollywood/2011/05/25-questions-for-boy-meets-worlds-rider-strong) interview:

Turner disappeared, you know. He had a motorcycle accident and we never explained what happened.

They only brought him in because Friends became a hit. And ABC was like, “We need twentysomethings on every show. Including on this kids’ show that’s only watched by teenagers.” And, remember, they had him and they had the character of Eli, who was the black young teacher. And they were going to have this whole twentysomething storyline.

I don’t know what they were thinking, but it was a complete disaster. It didn’t work at all.

It looks like ABC wanting BMW to compete with Friends is why the show developed the plot of Turner adopting Shawn. On The Dramatic Workshop Podcast (https://anchor.fm/scdwpodcast/episodes/Scene-53-Anthony-Tyler-Quinn-e1asnkm), Turner-actor Anthony Tyler Quinn says:

I remember, after the first season [Season 2], Michael [Jacobs, series creator] had come to me and said, "You're going to get Shawn. You're going to be his father. We're going to take it in that direction for the next season." I don't know if the network just... I don't know what their thinking was. It never quite materialized.

Perhaps towards the end of the third season, ABC declared that the Friends competition was a mistake; the demographics for BMW were ultimately family and adolescent viewers, not twentysomethings. ABC decided that Eli would disappear and the plot of Turner adopting Shawn was abruptly reversed with Shawn's father returning.

Also on The Dramatic Workshop Podcast, Anthony Tyler Quinn says he had known for all of Season 4 that it would be his last.

Michael [Jacobs] had told me in advance. My second season [Season 3] had just completed and that summer, he had called me and said:

"You're not going to be able to come back after this season [Season 4]. Because the network has decided that they want to do some things, change some things or whatever, and you're not coming back [for Season 5]. We're going to play out this season [Season 4], you're probably going to be sort of in limited use."

And when I read the ["Cult Fiction"] episode, I was kind of scratching my head. That's it? That's the wrapup? Just to leave it as a cliffhanger?

According to the BMW subreddit, Quinn must've signed a three season contract (with extension options). The plan was for Turner to become Shawn's legal guardian in which case BMW would have secured Quinn in an ironclad contract to ensure Quinn would be with the show long term. They would have done this with ABC's encouragement.

Then ABC changed their minds and wanted Quinn out, but there was no cause to fire Quinn. Had ABC released Quinn after Season 3, they would have owed Quinn partial payment for one more year. ABC didn't want to pay Quinn for episodes he wouldn't film, so they gave Quinn the contractual bare minimum of his Season 4 episodes and let his option lapse. (Again, it's my theory based on these quotes.)

Quinn says in the podcast that he and BMW creator Michael Jacobs remained good friends well after "Cult Fiction", so Turner's motorcycle accident as his last episode seems to me like an act of defiance from Michael Jacobs, almost as though Jacobs is saying to ABC:

"You want Turner out? Alright, here's his last episode! A cliffhanger! Ha! Now you have to let us bring him back! Viewers will be screaming for more Turner!"

But Quinn was working on other shows during Season 5. Maybe every time BMW had a slot for Turner, Quinn wasn't free and every time Quinn was free, BMW's schedule wasn't. Or maybe ABC was unmoved by "Cult Fiction" and wanted Quinn gone, cliffhanger or no cliffhanger.

Eventually, BMW accepted that the time to bring Turner back had passed. They had a returning Minkus call out to Turner on the "other side" of the school to confirm that Turner was alive and well enough to teach again (offscreen) and moved on.