Nowadays most of the apps with extensive features also end up launching a lite version of the app, and this is the exact same case with Facebook.
Facebook launched its Lite version i 2015 and a lite version of Messenger app was also launched a year later in few of the countries like Kenya, Malaysia, Sri Lanka. These apps were for the devices that were not able to handle original Facebook apps, particularly low spec smartphones and countries where getting a good connection is a luxury.
Now Facebook Messenger Lite is launched in many new countries, and you just need to head out to playstore to check it out. But it still leaves many countries and they tend to be a huge market for Facebook. Some of the excluded countries include India, United States, Brazil, China, Indonesia, New Zealand, and the UK.
Still, people can download the APK files manually and install the app.
Facebook took this step to lower the use of resources, as original Facebook app usually take up a lot of resources and drains the battery quickly. The Lite version is also going to lower the data usage while using the app.