For a non-violent crime which saw the recovery of a large fraction of the loot that's not 'only'.

It is relative to the other participants.

That's true, but likely factors were: they were in an adversarial power relationship with some of the other participants (notably: Madoff himself who repeatedly assured them that he actually made the trades that they reported but I fail to see how they could have been so gullible) and may have been put under significant pressure and it likely is a first offense all of which would result in a lesser sentence. But it still is obviously a very serious transgression, and none of their claims that they didn't know what was really going on seem to have fooled the jury.

Even the most kind reading of the situation leaves little doubt that they knew exactly what was going on.

Perhaps a small upside to computer programmers / techs often being viewed as low status compared to other highly paid professionals. :)