Why is the bottom lane called the safelane?

I think there are a few reasons with these two being the main ones:

First is that the safelane generally has the creep closer to their tower. They did change the location where the creep meet in 6.80 (I think?) by making safelane creep walk faster and longlane creep walk slower but it's still an advantage to the safelane. This makes last hitting much safer.

In the following picture, you can see the point where the creeps meet up without outside influence. Notice that the creeps meet up very close to the dire tower on the top left and radiant tower on the bottom right. You can further cause the opponent to overextend by pulling from a camp (red arrow):

Second being that you have your jungle as kind of a buffer between you and the enemy. You can move away from the tower with a lot more safety if you are in your safelane. The person in the offlane as soon as he goes past that river is in danger zone. He can easily get enemy supports right behind him that come from the jungle. The safelane has room for your supports to be near and still get xp by stacking and pulling or kind of just being near.

Some content borrowed from Z's answer here.