From the interview:
"So, why did I choose to do a portable game in the first place? This is also simple: to challenge myself. You might ask why I must do so, but that question would be foolish. Any creative endeavor must begin with a challenge. It took me 10 years to make the Dead or Alive series the number one fighting game on the planet. Making Ninja Gaiden the world's finest action game was a bit quicker, yet it still took four years. There are obviously many reasons why I was able to create these two franchises. One thing that I can say is that, as a group of people that love to challenge ourselves, the hurdles that I set for our team when making these games were so high that we couldn't help but be thrilled, and as a result the development process for each game was always very challenging."
(Please click the source link for the complete interview.)