why Facebook forced people to use the messenger app

1 To Simplify message sending

According to Mark Zuckerberg Facebook wanted to simplify the steps of sending a message so it created a separate App.
2 Mark thinks each App can do one thing well

Mark Zuckerberg said that he thinks that each App can do one thing well and by moving messages to separate App the experience can be improved. See why Facebook is so addictive.
3 Messaging is a very important feature of Facebook

Mark Zuckerberg said that 10 billion messages are being sent everyday from Facebook and that messaging is a very important feature of Facebook.
4 All other messaging Apps are Focused

Mark Zuckerberg said that all other messaging Apps in the market are focused on messages and so he believes this is the right way for Facebook users to send messages. See also why Facebook bought whatsApp
5 To provide services through a new platform

Some analysts suggested that Facebook is trying to imitate the move of some Apps , like line, which provide different services through it's messaging platform. For that to happen messaging need to have its separate platform
6 To reach a conversation faster

Mark said that when a person used to get a message on Facebook App he had to first load the App then reach the messaging tab. He said that the new App makes this process much shorter
7 Why Facebook forced people to get it

According to Mark Zuckerberg if only some people downloaded messenger while some others didn't then the messages might be delayed since the people without messenger might take longer to respond