They lack any meaningful design in comparison to other currencies, since all denominations stick to mainly black and white designs with slight hints of color as they've remained the same for a long time. Why won't USD switch to Polymer bills (it was first unveiled in Australia) like they've done in the UK or UAE for example.
Infrastructure costs would be notable. Vending machines, ATMs, and other money-receiving machines are calibrated for current cotten/linen bills.
Impetus. Changing to polymer bills would take political will and we can't manage anything productive atm.
Personally, I don't want Trump to drive the change because he would end up putting himself on the bills, and other distasteful characters from history.
save