Why is it not revealed what Jesse wrote in the letter addressed to Brock?

Originally the contents of the letter were a voice-over during the final images:

Originally, in the first draft of the script, that’s how the story ended. With Jesse driving through Alaska, and you hear the voiceover of what’s inside of the letter.

Aaron Paul elaborated further to TV Guide:

That was the very first thing that Vince Gilligan wrote when he wrote the script. He wrote that letter and then he wrote the script. In the original draft of the script, that is how the entire movie ends — the voiceover of the letter. But that was sadly cut. ... [It is] the most honest, beautiful, caring letter imaginable — really, just pouring his heart out and saying he's sorry.

And to IGN he revealed even more of its contents:

Actor Aaron Paul told IGN at the press junket for El Camino that he knew what Jesse’s letter to Brock said, describing it as "a beautifully honest, heart-wrenching sort of love letter to Andrea and to Brock. A confession in many ways."

In that same article Paul also revealed why the contents weren't used:

The actor added the letter "was very hard to read. And hopefully, knock on wood, Vince will allow me to release it to the world one day. I fought very hard to have it still in this film, but he wanted to leave it to mystery.”