...and why exactly told you that the PSP did "badly"?
According to wikipedia, as of September 2011, it sold 71,4 million units worldwide and it still sells. It's been a year since then and I can't be bothered to check, but I think that with its weekly sales results, pretty soon it's going to match, if not top the GBA's result of 81,51 million, which was considered legendary.
It's not the PSP that's at fault here - if anything, it's the DS that sells in unreasonable quantities. That, and it's also kind of unfair to merge DSi and DS sales as they are two entirely different console families, but I digress. In any case, Nintendo gave good reasons to switch from the DS to the DSi - DSiWare, SD card slot, cameras, all that jazz. A lot of people swapped, thus boosting the overall result. With the PSP, you bought one and you didn't need to buy another - there was never any groundbreaking innovation or new, exclusive titles for the revised version like in the case of the DSi, and with little incentive to upgrade, the PSP sales were lower than those of the DS - at least that's my little theory.
My point is, PSP's sales are not bad - in fact, they're quite spectacular, seeing that it was Sony's first attempt at a portable console.