The argument is that we pay taxes into a general fund, and can't prove how much, if anything, each of us individually would pay for abortions.
It's not like voluntarily earmarking $1.00 of our taxes to a Presidential campaign account, which we have the option of doing on our Federal Income Tax returns every year. IMO, governmental pro-aborts would be too nervous to do that. Never give the people a choice, when one has the power to grab whatever is desired.