In a piston engine, the cylinder walls and the sides of the piston form the seal that keeps the combustion contained. You have a lot of metal-oil-metal surface area to work with here, so it's relatively easy to get a good seal.
On a rotary engine, you have comparatively thin apex seals. These wear out quickly, and make it more difficult to get a good seal. Additionally, combustion always happens on the same side of the engine housing, so you tend to get warping, which also makes the sealing problems worse.
Reciprocating engines definitely have their own problems though (e.g. valves), and here I think you can safely argue that our longer history with them has helped a lot.
In a piston engine, the cylinder walls and the sides of the piston form the seal that keeps the combustion contained. You have a lot of metal-oil-metal surface area to work with here, so it's relatively easy to get a good seal.
On a rotary engine, you have comparatively thin apex seals. These wear out quickly, and make it more difficult to get a good seal. Additionally, combustion always happens on the same side of the engine housing, so you tend to get warping, which also makes the sealing problems worse.
Reciprocating engines definitely have their own problems though (e.g. valves), and here I think you can safely argue that our longer history with them has helped a lot.