Arbitrary choice, it's unlikely satoshi performed research about effect block propagation on different block time.
Partly that, but in the whitepaper, Satoshi did mention it as a measurement for calculation of RAM/disk space requirements. It can be that as arbitrary as "10 minute block time "is believed to be, it might really be based on the current hardware available of that time when he wrote the whitepaper.
It's plausible explanation, but difference between best case (block header with no TX) and average storage/RAM in 2008 is too wide.