I think it's because of who they are compared against.
N-Gage version of a game is usually vastly superior to any other mobile version.
DS version of a game is usually vastly inferior to a (potential) PSP version.
This is traditional games are not favorable to the reputation of DS, they underline the lack of processing power. Instead, games like Yoshi's Touch & Go, Nintendogs, Electroplankton, Zoo Keeper, Meteos, Wario Ware Touched etc. work like a charm on DS and won't get compared to PSP.