After 1.e4 c6, 2. d4 d5, the move 3. Nc3 is called the "mainline". According to lichess's database, the move 3. e5 has been played more than 3. Nc3, both by masters and by non-masters. This makes me wonder why we don't call 3. e5 the "mainline".
In general, what makes an opening move the "mainline"? Is it not the most popular move?