Only the continuous is possible there (if I take your meaning), because 'since last year' is a period of time and a promise is an instantaneous event. Compare: He had been promising to write since last year . = repeated promises from that year till the letter received.
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Mister MicawberOnly the continuous is possible there (if I take your meaning), because 'since last year' is a period of time and a promise is an instantaneous event. Compare:He had been promising to write since last year. = repeated promises from that year till the letter received.He had promised since last year = one promise given at one time after that year.