Even though the Wii U was as powerful as the PS3 and 360, it's having to compete against consoles that have sold 80 million units a piece. The die hard Nintendo fans are well known not to purchase much more than first party Nintendo games and the rare exclusive, and those that are multiconsole owners already have dozens of games on the other two consoles so they use them as their primary consoles.
If the NX is a console only and it's as powerful as the PS4, they're going to be up against two consoles that have sold 40 million and 20 million. The only plus side is that the games on the Xbox One and PS4 are finally starting to pick up so in terms of quality games, the NX won't be too far behind. Atleast with the portable console hybrid with the power of the Wii U, they can sell to two markets, a market that wants handheld games, and a market that wants console games. The developers will be able to develop one game and capitalize on both markets.