Mercer Inc. and Callan Associates Inc.‘s proposed marriage, announced with fanfare on Feb. 17, has devolved into a guessing game on why the firms abruptly killed their engagement a scant six weeks later. Industry observers who suspect Mercer fled the altar say potential legal land mines could have soured Mercer’s New York-based parent, Marsh & McLennan Cos., on the deal. At the same time, those who believe Callan rejected Mercer’s embrace point to signs that a significant number of both Callan’s clients and its consulting professionals were unhappy with the prospect of the San Francisco-based firm being subsumed within Mercer.