Very often, non-standard forms can be attributed to laziness.I find it surprising in this case as it seems so much easier to pronounce "an orange" than to say "a orange".
I feel the same about the increasingly common glottal stop in place of intervocalic /t/ and, for some speakers, /p/ and /k/. It seems to me that the original plosive requires marginally less micro-effort than the glottal stop, but it may just be that nearly seven decades of using the 'correct' (;-)) form makes me imagine it's easier.