Here is the why...
New reefers have a tendency to have messed up tanks because they are learning. Such as water off, like SG too high, too low. algae blooms and dosing the tank because they really don't know what caused the problem in the first place...on and on the list grows. It is a hands on hobby, and we all make our mistakes.
The ocean is an environment that has been stable for thousands of years. The creatures that live there have had no change and have not learned to adapt, because they never had to.
To survive when they are moved to a fish tank, they need to exist in an environment as close to the one they have adapted to over the centuries. Some corals are tank raised and are more forgiving...they are used to changes. Some corals just adapt faster and can handle changes easier.
These are the corals for the beginners because the tanks they keep are a learning tool for now. So like allot of us..start with an easy coral...keep the water as near perfect as you can...if there is a mistake it can usually be corrected without loss. Then one day you will realize that you have had near perfect conditions for a while...you can then move to a more difficult challenge to keep with confidence.
There will still be losses, it is inevitable…don't feel bad...even so called expert level hobbyists have crashes and lose everything. They just painfully know why.