Following the demise of Nadav and Avihu, Moshe addresses Elazar and Itamar as, “נותרים”, “who were left alive”. (10:12,16) Rashi tells us that following the Golden Calf debacle, Aharon was threatened with “destruction”, which would mean taking his sons. Moshe’s prayer saved 2 of them. Apparently the “אש זרה” came on the heels of a previous precarious predicament.
The Avnei Nezer (cited in מעיינה של תורה) offers an alternate suggestion.
ביום ההוא יהיה ה’ צבקות לעטרת צבי ולצפירת תפארה לשאר עמו …למי שמשים עצמו כשירים
Hashem will crown the righteous in the future times, specifically to one who humbly establishes himself as a “remnant”. (Sanderdin 111b) Elazar and Itamar viewed themselves as remnants, connoting humility. Nadav and Avihu were overly confident, as they rendered Halachik decisions in front of their masters (Rashi). Humility and closeness to Hashem are a good match.
Shabbat Shalom
