The explanation according to https://web.archive.org/web/20010221204734/http://sd.wareonearth.com/~phil/jumbo.html https://web.archive.org/web/20010221204734/ht

The explanation according to https://web.archive.org/web/20010221204734/http://sd.wareonearth.com/~phil/jumbo.html https://web.archive.org/web/20010221204734/http://sd.wareone... is: "First because ethernet uses a 32 bit CRC that loses its effectiveness above about 12000 bytes. And secondly, 9000 was large enough to carry an 8 KB application datagram (e.g. NFS) plus packet header overhead." That is, 9000 is the first multiple of 1500 which can carry an 8192-byte NFS packet (plus headers), while still being small enough that the Ethernet CRC has a good probability to detect errors.