The situation this year was a little different though. First, they updated the MBPs in October 2016, which would have made a very narrow time frame for the next update if they put it out in march.
Second, the improvements of the 7th gen chips over the 6th gen chips were tiny, so it didn't really matter if the competition used the newer CPUs while Apple kept selling notebooks with the older CPUs – this time, it's different, with more than 40% performance increase in many workloads.
Third, a March event is relatively likely. Apple had a march press conference in 2011, 2012, 2015 and 2016, just to mention a few.
Fourth, there are more products Apple most likely wants to announce around this timeframe, like their new wireless charging mat, the HomePod (again), possibly a lower end iPhone SE successor and maybe also a new Mac Mini and Mac Pro.
Sure, they could just dump it all into WWDC, but for the MacBook Pro line-up, I feel that the competition is hard to ignore, with notebooks which are - in some cases - almost twice as fast as Apple's own offerings.