It's the benefit of hindsight. Cartridges were really expensive to produce, and there wasn't always a guarantee that they were going to sell. I mean, Nintendo officially translated Mother 1, and then never released it because they didn't think it would make back the manufacturing and marketing budget.
At the time, Nintendo's target market demographic was young boys. Picross is a game that appeals mainly to adults; only a few kids I know would have had the patience to sit down and do a logic puzzle. It's not like Tetris or Dr. Mario, which have a bunch of micros successes every few seconds, building to a larger success at the end. Sometimes, a single Picross puzzle can take you 30 minutes, with a payoff only happening at the end. As much as I absolutely love Picross now, I think it would have been a risky endeavor to release it to the US at the time. That's just my 2 cents though.