Perhaps he was also keen to avoid Mrs Bennet getting carried away with the possibility of Mr Collins marrying one of their daughters, as is hinted at in the letter - "my readiness to make every possible amends". I suspect that Mrs B would not fail to see the potential of this being a satisfactory match in her eyes. Was Mr B keeping quiet to preserve some peace and sanity until the last moment, especially as he doesn't hold much hope of Mr Collins' being the sort of character he'd desire for a son-in-law!?