Because the Snow on Mars is CO2, not H2O
H2O is a tiny trace component in both the atmosphere and the seasonal snowpack. It is present at about the 0.1% level or less
Only in the thick polar ice caps is there a large concentration of H2O "snow" and most of that is welded into an icecap.
There is plenty of frozen subsirface H2O, but for the avalanche mechanism, you need seasonal snowpack and ablation.
Anyway, if H2O ice-related avalanches were responsible for the gullies on Mars, there would be direct analogues on Earth. Nobody has shown me any yet.