What is the reason for these changes in Game of Thrones from book series to TV Show?

Your premise is flawed.

Tyrion's actual quote is as follows:

You poor stupid blind crippled fool. Must I spell every little thing out for you? Very well. Cersei is a lying whore, she’s been fucking Lancel and Osmund Kettleblack and probably Moon Boy for all I know.

We, and Tyrion, actually know for sure that she slept with Lancel. And that's part of the TV show as well. But we are never told that she actually slept with Osmund or Moonboy. Or anyone else for that matter. That's probably just an angry Tyrion lashing out at Jaime. The only person we know for sure that she slept with in that period was Lancel. She does flirt a lot with the men she wants to control, but we see glimpses of that in the show as well.