Why was the search bar moved to the bottom of the screen?

This is a user to user only forum. No one posting here works for or is in any way associated with Apple. As such, we can't answer "why" questions here at all. We can guess, however. And for years, people who use their phones on handed complained to Apple using this link --> Feedback - iPhone - Apple that having the search bar on the top of the screen made it harder to use their phones one handed. Again, that's a guess as I too don't work for Apple.

Motor memory can be easily retrained. I no longer reach for the top of my iPhone screen when I want to search for something as I now have become very adjusted to the search bar on the bottom of the screen.

You can use the same link to let Apple know your thoughts as it's the only way they'll see any feedback you may have.

Just for your general knowledge many people use their iPhones one hand and the thumb or forefinger except on very large hands can’t reach that far. In the past many users requested the location change.

Any listens to customer feedback and it works. Just look at the reason we’re all posting. Follow the link lobsterghost1 provided and give feedback on the changes you’d like.