Er because their whole religion is found and supported on the idea that the GB are in God's hands.
Most of the JW doctrine is an interpretation of scripture, that interpretation coming from the GB.
They must have authority in the rank file members eyes and 607 > 1914 > 1919 gives the GB that self-appointed authority.
If you remove 607 you remove that authority then all their doctrines are interpretations of men, not God appointed.
JW's cannot accept that, cognitive dissonance.
The whole reason why many of us have left is simply down to the fact we realize that the GB are not appointed by a "God" and are indeed self-appointed.
This is why when people leave for other reasons and do not research and still "believe" that the GB are God's spokespeople that they will either go back or their conscience cripples them to mental health issues.
607 BCE as the destruction of Jerusalem is the absolute core and foundation of JW's because it then gives credence in their eyes that all other doctrines are from "God".
Ironically, 607 is one of the easier doctrines they profess to disprove.