Health Care and Money.
I know many smart people who'd do non-full time work if they had healthcare provided. But, unless you're a FTE you're not getting that.
Also, the pay differences is pretty low. Why would I work as a Dev at "Random Contract Company" that is offering $65-75/hr when I could just get a FT job that pays $140-150k/yr and includes PTO and other perks?
If part time/contract paid as much of a difference as travel nurses for instance, you'd see a lot more contractors.