Because HTTP is simpler, faster, easier, more reliable.
The motivation for a a lot of p2p is to make it harder to shut down, but there is no danger of that for Linux distros. The other would be to save money, but Debian/Arch/etc. get more than enough bandwidth/server donations, so they're not paying for that anyway.
Input/output error is very weird, maybe you got file corruption?
Does
dmesg
show any errors with the disk or file system?If you run
you might be able to figure which file it is that cannot be read.