Learn Russian: Из-за аллергии мы купили свежие фрукты вместо этих орехов. - Because of the allergy, we bought fresh fruits instead of these nuts.

The preposition вместо (instead of) requires the noun that follows it to be in the Genitive case. Because we are talking about multiple nuts, we use the Genitive plural forms: the demonstrative pronoun эти becomes этих, and the noun орехи becomes орехов.

Why use из-за for "because of" instead of something like потому что?

Потому что translates to "because" and is used to connect two full clauses (e.g., "because she has an allergy"). When you want to say "because of" followed directly by a noun, you use из-за. Like вместо, из-за also triggers the Genitive case, which is why аллергия becomes аллергии.

Why doesn't свежие фрукты change its ending if it's the direct object being bought?

It actually is in the Accusative case here! In Russian, inanimate plural nouns (and their matching adjectives) have the exact same endings in the Accusative case as they do in the Nominative case. Because fruit is inanimate, свежие фрукты looks just like the dictionary form.

How is the Genitive plural орехов formed?