To follow up on what mrv said:

1) Disc brakes don’t work with track dropouts, because the wheel can only be in one position relative to the caliper which is mounted on the chainstay. With rim brakes, the brake pads are moved up or down to align with the rim as the wheel position is adjusted in the dropout to achieve the proper chain tension.

2) Disc brakes are a lot heavier than rim brakes.

3) You have to place a wedge in between the brake pads when transporting the bike with the wheel(s) removed, otherwise the caliper pistons can be pushed out if you accidentally depress the brake lever.

4) Most disc brakes are hydraulic, and setting them up requires bleeding air from the brake lines and properly filling the master cylinder reservoir with brake fluid. Also, if the brake lines get cracked or damaged for any reason, you instantly lose all braking function.