As more income in our economy has shifted to upper brackets and as income that's not subject to Social Security withholding. Furthermore, capital gains (income generated through investment) are not subject to withholding.
Combined with the Bush tax giveaways, the very wealthy pay an overall effective tax rate that is less than middle and lower income wage earners.
It's led to the Social Security being in trouble long-term. Instead of cutting benefits, or increasing the retirement age simply remove the cap (currently the first $106,800 wages) & subject ALL income to withholding.
Lifting the cap & subjecting capital gains to FICA withholding would increase the revenue enough to make it not only permanently solvent but would run at a profit & allow benefits to be increased (for everyone).