There seem to be eleventy-zillion reasons not to change who's on any given denomination of money, and they all boil down to two:
1. "Person X currently on the $Z bill is a great American icon and it would be disrespecting him to boot him to the numismatic curb."
2. "Person Y being proposed for the $Z bill was a commie / war-monger / killer of babies / killer of someone / corporate shill / environmentalist fanatic / religious fanatic / atheist / person of the wrong race or ethnicity or religion or gender or orientation / person we cannot abide the immense honor of being on our money."
The last point is the biggest — really, nobody's going to kvetch too much over Hamilton, Jackson, Grant, or Franklin being moved aside (Washington and Lincoln are probably too iconic). Everyone and every group of everyones has people they must have on the money and people they mustn't have on the money, and the Treasury is highly unlikely to do anything unless (as the article notes) somehow it improves anti-counterfeiting measures.
That's the one weakness in this particular campaign: that it is looking for a singular change, and not proposing a regular rotation. People might be willing to put Sanger on the $20 if they knew that in two years they could be collecting Reagan double-sawbucks. A rotation between Teddy, Franklin, and Eleanor Roosevelts could be arranged.
(I still stand by my position, though, that the parties involved should not only not be living, as is current policy, but should have been dead at least 75 years; that gets around a lot of the living memory political brouhaha, even if it excludes a lot of neat people.)
But going for a single person — especially a woman (which some yahoos will resent already, particularly since any woman who's made it to prominence enough to warrant the honor is likely to have ruffled feathers in some fashion) — simply makes it that much mroe of a high stakes thing. "Do we want Ms. X on our $20 bill forever?" The fact is, there are very, very few people who I want on our $20 bill forever — but that seems to be what we are stuck with.
Of course, if we keep debating long enough, we may get to the point where most folk say, "$20 bills … do they still make those?"
How Hard Would It Be to Change the $20? — NYMag
The Treasury hasn’t subbed out a portrait on a dollar bill since 1929.