Ip is by default in /bin, perhaps because it's more "modern". Ifconfig
has always been in /sbin, long before Debian existed.
The exact reasons are in the mists of
time, but it seems likely the powers didn't want users to
routinely use programs better run by adminstrators.
Using sudo automatically gives you the
root path so you can run programs 'better run by admins' without
extra work figuring out paths and any local variations.
In some cases sudo is actually required
and in some cases it makes no difference.