This is actually less than I expected. Sonic Generations sold around 2m in the same amount of time, and it was more expensive on release than Mania was. Of course Generations had a physical release and Mania didn't. Hopefully the Mania physical release sells well this Summer.
Generations was designed to have a much wider appeal, Mania is a 2d game with Sega Genesis graphics, it just doesn't have a wide enough appeal.
Which is why even forces has sold more, sadly.