Port, older style, high price, bad misjudgment on demand from Rockstar... CTW never looked like shifting much on PSP.
Certainly despite having both previous PSP GTA efforts (neither fully finished admittedly) I had no interest in this - although, to be fair I have a DS and had no interest on it for DS either (like many others apparently).
I suppose, given the 360 exclusive episodes, there is also a small chance of negative response from PSP owners, particularly if they have a PS3 as well, to another GTA title - although the sales of LCS would seem to disprove that as a possibility.
Try to be reasonable... its easier than you think...