Learn Dutch: Sommige consumenten kopen dit merk omdat het erg populair is. - Some consumers buy this brand because it is very popular.

Because it is followed by the noun it modifies (consumenten). You only add the -n to make it sommigen when it stands completely on its own without a noun AND refers to people (for example: Sommigen kopen dit merk, meaning "Some buy this brand").

Why do we use dit merk instead of deze merk?

Merk is a het-word (het merk). When you want to say "this" with a singular het-word, you must use dit. You only use deze for de-words and all plurals.

Why do we use het to say "it is very popular"?

The word het here refers back to dit merk. Because merk is a het-word, we have to use the pronoun het (it) to refer to it. If the brand had been a de-word, we would actually refer back to it using hij or zij instead!

Why is the verb is at the very end of the sentence?