Let me try to explain.
(Please note, my explanation is not an accurate dictionary explanation. I only intend to explain the choice of answer here.)
忽视: not giving any emphasis or enough emphasis (either intentionally or unintentionally)
我们现在已不能忽视这么严重的问题了 (they did not pay enough emphasis before)
忽略: not paying attention (either intentionally or unintentionally)
我们现在已不能忽略这么严重的问题了 (it sounds like they did not look at or even think about the question before.)
I think on some occasions they can be used interchangeably, but not here. In this case if you use 忽略 the meaning will change into another sense.
