No "SSTORE" opcode found in the result of debug_traceTransaction()

Question

when I try

'curl -H 'Content-Type: application/json' --data '{"jsonrpc":"2.0", "id": 1, "method": "debug_traceTransaction", "params": [ tx_hash ] }'

, the result does not contain "SSTORE" opcode but I don't know why.

Besides, the number of "op" is 38. I wonder if the number is too small.