The area of Long Island Sound where the accident occurred can have a fair amount of backlighting from the shorelines (there are two; the Connecticut shore to the North and the New York shore to the South). It can be very hard at times to pick up navigation lights or government markers. The masts are invisible at night (If the sails are also up, they can be visible, but even in that case, somebody needs to shine a light on them to make them visible). Radar can be tricky; the collier probably presented a good target, while the sailboat (even a large one) may or may not. But what was the radar range setting? If set at 12 miles in Long Island Sound, it is of limited value, since both shorelines are the main reflection (it looks like you are going through a lit-up funnel).
There are so many possibilities; we will have to see if any further facts are made public.