From today's Boston.com
Apparently the clock was running and the Seahawks had their passing unit on the field. To put in subs they'd have to call a time out.
"Of course, as is everything, it was calculated. Here, many coaches would zig—in this case, call a timeout and preserve time for a game-tying field goal drive. Belichick instead zagged, letting the Seahawks run out the clock so their best player would only have at most two shots at the end zone. With one timeout, 30 seconds and a four-point deficit, one stuffed run could well have spelled defeat for the Seahawks.
Post-game, Pete Carroll defended the decision to pass, saying his team’s three-wide set was a poor match for the Patriots’ goal-line unit. And while it’s been poo-pooed as his falling on the sword, it’s also the truth: even Lynch would have struggled with so few blockers. Thus, the decision was thether to burn the timeout and sub, or throw a pass."