You must: a) Be a Picross Addict and b) Take the free-to-start aspect as a personal challenge
I have a Picross problem (played all other Nintendo releases), and I'm actually on the positive side of the fence, in favor of Pokemon Picross. I've been playing it since release, and I surprisingly like that the Picrite economy forces me to play a short daily training every day. And the fact that I have to beat the challenge time to get an extra two currency has made me get faster at completing puzzles. It's a nice, long, daily drip feed of Picross with the occasional day or two where I've had enough money to buy an eight-ball of puzzles where I can go into a frenzy, much like a drug addict.
I will admit that some days it takes a HEAVY amount of restraint to not spend the full $30 of actual money (the technical full purchase price) on the game, but I think the fun truly comes out of trying to complete the achievements and level challenges to squeeze every drop of currency into buying the next set of puzzles. I will admit to paying $5 for the two "on sale/best value" Picrite packs, mostly to get the Mega Pencil, since saving up 500 Picrites is insane. I can easily justify spending $1 less than I spent on one Picross E set.
Oh, and there are Pokemon.