I think a better question would be, why does anyone say all three paragraphs? After all, the Shulchan Aruch (OC 239:1) writes to only say the first paragraph!

The answer is that people who tend to pray Ma'ariv before Halachic nightfall (according to many opinions) should read all three paragraphs when it is definitely nightfall. An opportune time to do this is when saying Keri'as Shema at bedtime (and remembering to have in mind to fulfill the Mitzvah of Keri'as Shema). However, people who usually pray Ma'ariv when it is definitely nightfall have no Halachic reason to read all three paragraphs. Some do so anyway for various reasons, but this is neither a requirement mid'Rabanan nor a mandatory custom.

It is possible you have been observing that many Sefardim have the custom to read all three paragraphs because they often recite Ma'ariv early, while many Ashkenazim do not do so because they generally do not recite Ma'ariv early.

All the best,

Yaakov Montrose