drewcoul - I think they say that for two reasons.
Firstly, it interferes with the 1914 extrapolation.
Secondly, they claim that 587 contradicts the "seventy year" exile of the Jews. If the Jews returned home in 537 (or thereabouts), it is claimed that the exile must automatically have begun 70 years prior to that. However, there are a number of lines of argument to suggest that it isn't that simple. For one thing, Jeremiah's original prophecy was that Israel AND the surrounding nations would be in servitude for seventy years, and NOT NECESSARILY exile beyond their borders. Also, numbers are deeply symbolic, and the word "seventy" must be approached from an ancient Jewish perspective, and not from a modern western date-obsessed perspective.
I wish I could give a more detailed answer, but I have yet to throughly scrutinize the excellent "Critique of When Was Ancient Jeruslaem Destroyed" by Doug Mason (PM me if you want a link to the download). The truth is, I can't get excited enough about the subject because I no longer believe in the 1914 chronology anyway.
Hope that is helpful.