Mirrorless is incredibly common. Every major player has a full range of mirrorless options. Pentax is the last holdout not offering a mirrorless option.

The last market segment that was holding out was the Pro and Prosumer segment since initially mirrorless lagged on feature parity, especially in AF. Mirrorless cameras have effectively reached parity now.

Now there is, in real use, no advantage to SLR type cameras, and, pure guess, mirrorless are probably cheaper to make since you can dispense with an entire mechanical assembly.