Why do some wines and not others attract fruit flies?

When my friends and I last went camping each of us brought a variety of different bottles of wine to try.

Each evening we opened a different bottle and some nights the wine seemed to lure fruit flies while other nights nary a fly was to be seen.

The flies did not seem to have a preference as far as white or red nor were they elitists or snobs as they chose to haunt wine from both low and high price ranges.

Any ideas on what it was that attracted them?

By the way any guess is welcome- even if in humor.

:)