One theory I've heard is that one of the wives of Noah's sons, possibly Hams, carried the Nephilum gene onto the ark. Maybe one of the reasons Hams son was cursed to the degree he was by Noah.Do we know that He has prohibited it since the Flood? I think He probably has, but don't know where the Bible makes that clear.
The post Flood presence of giants in the Bible makes me wonder. Og king of Bashan and Goliath are two examples.
Not my theory, just something I've read.