Mostly for the same reason that you only see 2 semi-auto .22mag rifles (Volquartsen and Magnum Research) on the market. The pressure curve of the .22mag (and .17HMR) are not well suited to the standard blow-back operation used in semi-auto rimfires. The pressure spikes late, after the bolt has come out of battery, can and has blown cases in other designs. The Volquartsen and Magnum Research rifles use a spring loaded gas buffer to equalize the pressure and slightly delay the blowback.