Because if you look at the two as a non educated consumer, they practically appear to be the same. Its the same product, same concept, same mass appeal.
Also look at this:
Piracy for PSP titles according to numbers some people "produced" - probably by skimming torrent tracker data, was about twice, to three times the issue on PSP, than it was on the DS.
Now correlate this with the fact, that PSP games where much larger, so torrent use would have been more prevelant, as supposed to DDL - which had to skew the data, so lets lower that percentage.
Then compare overall sales figures with the DS, and a more congruent image starts to emerge. So even though piracy was an undeniable issue - it didn't boost general audience, or high visibility title sales even close to DS figures.
Seems like people didn't take to the system, if you ask me.
But of course not, if you ask the industry:
http://www.siliconera.com/postgallery/?p_gal=73937|4
(Also the source of the graph above..)
edit: Also, if you believe that data, PSP downloads surpassed PC piracy numbers:
https://torrentfreak.com/the-most-pirated-games-of-2009-091227/
- by a factor of 2x on even the most niche titles, compaired to the most popular PC titles at the time - that also werent very successfull going forward (Phantay Star now is a dead franchise, Dissida had unsuccessfull sequels...), which seems a little odd...
edit2:
Torrentfreak has top PSP piracy DL data according to their metric for 2008 as well:
https://torrentfreak.com/psp-piracy-is-trending-up-despite-sonys-claims-081222/
Lets look at their metric:
Dissidia: Final Fantasy was downloaded 200,000 times via BitTorrent over the past 5 days, which is pretty significant for a PSP game. The two most pirated games for the PSP in 2008 were Final Fantasy VII and GTA Vice City Stories , with an estimated 650,000 and 550,000 downloads.
As a comparison, the average PSP game sells
133,000 copies.
(Average is an issue, if you compare it to the two top spot, but it seems to indicate a 2:1 ratio. Of pirated to bought.)
Lets take their Top 2 figures, and compare it to other console systems at the time (actually one year later, but hopefully that doesnt skew the metric too much).
Pirated copies in 2008 (PSP) and 2009 (all other systems) according to Torrentfreak torrent tracker data:
PSP
1
Final Fantasy VII (650,000)
2
GTA Vice City Stories (550,000)
Xbox 360:
1
Call of Duty: Modern Warfare 2 (970,000)
2
Street Fighter IV (840,000)
Wii:
1
New Super Mario Bros. (1,150,000)
2
Punch-Out!! (950,000)
PC Games:
1
Call of Duty: Modern Warfare 2 (4,100,000)
2
The Sims 3 (3,200,000)
This seems to be more congruent with general system sales at the time. Lets hope the time windows were comparable. Now with lower system sales - percentage wise PSP piracy has to have been "more of a problem", but to argue - that it "killed" the PSP, when Sony still put out four revisions of the hardware, trying to compete in a "lost" market - doesnt seem to be the correct conclusion.
In fact, as an absolute number PSP Piracy seemed to fall in line with other system numbers at the time. Now the question becomes, how much of Piracy in general is "opportunity driven". (So - "how much" would it have scaled with system sales, if Sony had sold more base units.)
edit3: Xbox 360 and PSP had comparable global lifetime sales for hardware, apparently (- so we dont even have to "scale up" the PSP numbers to do that comparison):
7
Xbox 360 (X360) 85.80 Million
[...]
9
PlayStation Portable (PSP) 80.82 Million
edit: NDS sales figures for comparison:
2
Nintendo DS (DS) 154.90 Million
http://www.vgchartz.com/analysis/platform_totals/
So lets put this into perspective again.
Torrentfreak data suggests, that PSP piracy was "at expected levels" for top titles. Industry data would like to indicate, that Piracy in selected niche series titles, which became derelicts in the future, was 4x to 7x higher than Torrentfreak indicated for the top titles on the system - which is very odd. But also, according to Torrentfreak, comparative values were in line with other console systems at the time. One of which had very similar lifetime sales.
The industrys public NDS piracy data we can safely ignore - as NDS games were populated via DDL at the time, which they had no real insight into.
(But I can certainly tell you how it felt at the time, when everyone with a Flashcard had 60 games on their NDS "to go" compared to 4 on the PSP, because of storage space limitations... It also didn't seem like NDS piracy was 10x less popular, because you had to order a flashcart. They were ubiquitous, so was their advertising.)