Simple enough.
There is no evidence
for it.
There are continuous records by both the Chinese and the Egyptians through the period it was meant to occur.
There is no layer of flood deposits in the right age range that extends globally. There are lots of flood deposits - we know what they look like - but they are small and isolated incidents, compared to a putative "great flood"
There is no evidence of a genetic bottleneck in all species around that period.
Whatever Woodmaroppe may say, the Ark is not feasable. Just ask a zookeeper.