Previously if you had an extreme opinion, you had to moderate it because others around in the real world most likely don't share the same opinion. This causes extreme opinions to stay at the fringes of society.
Now with the internet, you can find any community willing to share your extreme opinion, geographic closeness is no longer needed. Echo chambers then form and you feel like you're part of a community, and so no one moderates your opinions. If anything, opinions get even more extreme.
To add to that, internet also ensures all the extreme things are amplified because attention economy. So if previously you would read a local newspaper about worst cult in your city, now you read NYT about worst cult in the country.
Now with the internet, you can find any community willing to share your extreme opinion, geographic closeness is no longer needed. Echo chambers then form and you feel like you're part of a community, and so no one moderates your opinions. If anything, opinions get even more extreme.