A nuclear power plant failure, in the worst case, is much worse than the worst case pilot error.
The 737 was a smaller complement for the 707 and 727 when introduced, it was never the largest plane flying.
In any case, the 1960's foreign body impact regulation has not been updated so far as I know, and at the time it was designed for the largest civilian passenger aircraft then flying. The 737 fits in, the 747 (introduced only two years later if I'm not mistaken) does not.