It's been a long time since I was held responsible for this kind of chemistry judgements but here is my best guesses...
#1 The polyacrylic molecule is too big to come in close contact with the mineral component of the enamel and would therefore not be effective in "dissolving" out the non-organic portions of the enamel.
#2 Polyacrylic acid by its nature is more likely to bond itself to other chemicals then to "eat" other chemicals. So it makes for a bad etching solution.
I know that phosphoric acid is more commonly used, but I saw the polyacrylic acid used on occasion at the dentist I shadowed. Could it be used to etch composite or on top of a porcelain crown?