The Japanese gaming giant originally planned to deliver 64GB cartridges to developers by the second half of 2018. Sources say "technical issues" have pushed the delivery date back to 2019.
The delay reportedly disappointed some software makers, especially US-based publishers known for producing data-heavy games. Some publishers may even postpone the launch of their games on the Switch in order to have access to the larger cartridges.
Nintendo currently offers cartridges in capacities of 1GB, 2GB, 4GB, 8GB, 16GB and 32GB. The handheld itself only has 32GB of internal storage although some of that space is reserved for system software.
Cartridges are typically viewed as being more durable than optical discs and afford faster load times but they're also more expensive to manufacture and offer limited capacities (compared to discs).
Nintendo's Switch got off to a promising start. The handheld console launched in March but was plagued by inventory issues for much of the year. Nevertheless, Nintendo managed to sell more than 10 million units as of early December (that figure is likely a bit higher today given holiday sales).
At this rate, it's entirely possible that Nintendo will sell more Switch consoles in its first year than it did Wii U systems across its entire five-year run.