@Tearfulidea1
Groups translate and post the chapters on MangaDex, and typically at the end of the chapters, you will typically see the name of the group and the website, saying "Go read it here!" for Advertisement. However, since we are reading it on MangaDex, an ad free website, these groups do not get any money from ads or views. Some groups get butthurt that they can't get people to read on their website without an adblocker, so they say, "Hey! We are removing all our chapters on MangaDex! You can only read our translation on our website!" and remove the chapters.
Sometimes this means chapters 100+ are available but 1-99 are not, making you read the missing chapters somewhere else. (I don't read it on the scans website out of pure spite.) If someone re-translates the removed chapters, it's purely out of generosity.
TL;DR
Some groups get butthurt they can't make money on here so they make it so we can't read it on here. <3