Because a felony is damn easy to get and almost impossible to get rid of.
Pissed on a wall or mooned somebody when drunk in front of a cop? Felony AND sex offender--just for good measure.
Had sex with your 17-year-old girlfriend when you were 18? Felony although some jurisdictions make exceptions and try to reduce this to misdemeanor depending upon age difference.
Picked up for drunk and disorderly and had a knife on you that exceeds specifications? Felony. Side note: there is a reason so many people like MagLite flashlights--they are similar in size to a billy club or truncheon but are legal.
Picked up with a small amount of marijuana while your friend was in possession of a much larger amount in the car. Felony.
Every single male I know has done something sufficiently stupid when he was young that could have been a felony.
In addition, if a cop picks you up, they can probably find a felony somewhere in the law that will stick unless you are a teetotaller.
The real problem is that non-violent felonies need to go away after a while. I seem to remember some research showing that if a felon manages to stay out of trouble for 5 years he is less likely to get in trouble than even the general population.
So, to answer your question, felons who stay out of trouble for more than 5 years are actually a better pool to hire from than the general population.