I think it's really the price point that makes the big difference. Games on a phone are little more than a "Newgrounds"ish title that people are now willing to actually pay for. And I think as evidenced by the proliferation of the "microtransaction" model it's a lot easier to pull people in when they can pay a little at a time rather than all up front. Even though the $30 game probably will last longer and be more 'meaningful', people are more likely to buy six $5 games instead because they feel like they get more and they have to commit less at once.
The problem is most people look at a PSP/DS vs a phone and don't see a difference. To be fair when most people just want their facebook games and angry birds type timewasters, there isn't a difference, because they won't get anything out of the Golden Suns, the Castlevanias, the 'long form' handheld games. As such I think you're seeing customers who always wanted that 'distraction' out of a portable but wouldn't pay for it are now 'entering the market' and buying a couple games, and you also have customers who basically just wanted their Game Boy/PSP to kill time who are now switching to a more economical option in their phone.
Though, with that graph is I would question what 'qualifies' someone with a phone to be included, as I don't think that someone who just downloads Bejeweled and Angry Birds on their phone is necessarily part of the market that the PSP and DS appeal to. I think it if were to reflect 'revenue from users who spent at least $20/$50/x on "mobile gaming" ' that Nintendo's and Sony's market share would not drop nearly as sharply.