I'm sure there's other reasons but for a start both use the same /Altiris web address in IIS, which i don't think is configurable.
If you build a new 7.1 server, highly recommended anyway instead of reusing the existing v6.0 Server, they can still get packages from the old server. If you have to reuse the old hardware, consider virtualising the v6.0 NS. A phased migration from 6 to 7.1 is better than a "big bang" upgrade.