Turns out the reason is... genders. Baguette is feminine, sandwich is masculine, fuck, une is fucking feminine, so sandwich doesn't match because fuck.
Fuck genders.
(Not bagging on the language itself (mainly), just how genders can be frustrating in a whooooole neeeww waaayyy)
@Mycroft It's definitely very different from how I have learned languages anywhere else. And I am learning and it's fun and easy, so I guess it works. But I like more structure. I like vocab lists and conjugation tables and clearly defined rules. So I'll keep using Duolingo to practice and drive what I learn, but I plan to supplement with more traditional materials.
And then sometimes Duolingo is cool in that it just presents you with new words and just sees of you can guess based on context. That's neat. Sometimes it's easier to guess new words than others...