Because the tyres/downtube combo make it harder to lower the wheel enough to pull the fork out of the headtube.
Also with the wheel on, there isn't any way of moving it out of the way to get enough room for the steerer to clear the headtube.
@edscoble started
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Because the tyres/downtube combo make it harder to lower the wheel enough to pull the fork out of the headtube.
Also with the wheel on, there isn't any way of moving it out of the way to get enough room for the steerer to clear the headtube.