The important question is why Comey doesn't want to discuss, or even acknowledge, that fact.
The answer to that question is that discussing and acknowledging the irrefutable fact that the FBI spied on the Trump campaign leads into other discussions he finds even less desirable, such as whether the spying was legal -- "adequately predicated" -- and whether it was politically motivated (in a word, an attempted "coup").
Why doesn't Comey want those discussions? That question pretty much answers itself.
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