The reason for the delay is probably because it was done through a private lab. Private labs in Massachusetts take between 3-7 days to get your results back. State and federal labs are much faster.
This is not necessarily true. I have two friends in MA who received the test on Saturday. One was a presumed positive (symptomatic and exposure to confirmed Biogen cases), the other a healthcare worker. My friend with Biogen exposure got his through a private lab, and learned he was negative on Tuesday. My friend who is a health care worker had to get hers tested by the state — she is feeling better but is still quarantined, having yet to learn her results almost a week later.
Totally anecdotal, but enough to convince me that state labs are not necessarily quicker than private labs, at least in MA.