http://www.techradar.com/news/nintendo-is-reportedly-delaying-64gb-game-cartridges-until-2019

Bigger Nintendo Switch game cartridges means more data and less reliance on memory cards, but Nintendo is pushing back the roll-out of 64GB cartridges for the Switch until 2019, after originally planning to introduce them during the second half of next year.

That's according to "people familiar with the matter" speaking to The Wall Street Journal, so you're going to have to wait a few extra months for games with oodles of content to explore and play through. Apparently technical issues are causing the hold-up.