Bittorrent support is a legacy feature; I don't think I've seen Amazon advertise it for several years.
It seems like an arbitrary limit, but an adequate one for the time being.
If (for whatever reason) you choose to use S3 as your tracker, then you just need to commit to breaking your >5GB content lumps up into multiple pieces.
I have 60gb files in S3, and saturating available bandwidth to download them in a reasonable time frame if they're not in the same country is actually a bit of a challenge.
Bit torrent would be one of the fastest and fault tolerant ways to retrieve the files, so it not being available stings a bit.
While I'm trying to download large files on consumer internet, I'd imagine that huge files between geographic locations and server grade connections would face a similar problem.