There are still many machines, for examples, that running at offices or on XP etc and can't run Java 1.8
And I guess the number is significantly large.
NXT can run on both Java 1.7 and 1.8. No specific reason why NAS should limit to 1.8 machines only.
There is no reason to run NAS on Java 1.8 as far as the code & features go. The only reason we say it now is because Patapato compliled NAS 1.1.3N2 and 1.1.6N1 (NAS with NXT 1.1.6 patches) from his own system using Java 1.8 which makes it a requirement to run.
If you have Java 1.7 and don't want to re-compile the code, use NAS 1.1.3N1.
If you have Java 1.7 JDK and want to compile 1.1.6N1 yourself to run on 1.7, you can find the sources here -
https://bitbucket.org/patapato/nas/overview