576 bytes was the standard buffer size in DECnet Phase IV use inside DEC.
Why 576? Because a FILES-11 disk block was 512 bytes, you needed a few more bytes for protocol overhead, and the allocation granularity on a PDP-11 memory management unit was 64 bytes.
So, a 576-byte buffer allowed the transfer of whole disk blocks without the need to split and recombine the data into session protocol data units.
This in turn allowed the possibility of DMA from disk into the appropriate 'hole' in a file server's network buffer.
In short, it's all about efficiency.
Other networking protocols probably did not 'copy' DECnet, but I suppose that similar reasoning applies.