They kind of already did with the 3DS. OoT 3DS, Super Mario 3D Land, Smash Bros, Hyrule Warriors, Xenoblade, Luigi's Mansion... these are home console-type games, on a portable system. Nintendo didn't really try very hard to go after the DS market that bought games like Brain Training, Nintendogs, etc. That market had already abandoned them for smartphones.
That's why the Switch makes a lot of sense for Nintendo... they were already basically making the same video games twice, once for each system. The Wii U and 3DS versions of each game have the same basic gameplay and feel, so it's much more economical to just develop the one game and people can play it however they want.