Using formal language both in oral and written form is an advantage for you. You will be noticed by people and they will remember you more because your language stands out from the language use of others.
Using formal language infers that you are a well-educated and well-round person. People will likely believe in what you say because it will give credence to you and your abilities.
However, when you use formal language with people who are not as well verse to its usage, you will face discrimination or ill-will because they will feel insecure and intimidated by you.
There are several different reasons depending on the perspective you look at it.
If you're looking at it from the government perspective, then this was due to the fact that they wanted to get rid of the tradition and have a complete take over of the culture by getting rid of it.
Other explanations are that they wanted to switch an alphabet or genuinely wanted to simplify the actual characters.
Hope this helps!
