I think the answer lies in the Silmarillion in Manwe's words:
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In this matter the power of doom is given to me. The peril that he [Earendil] ventured for love of the Two Kindreds shall not fall upon Earendil, nor shall it fall upon Elwing his wife, who entered into peril for love of him; but they shall not walk ever among Elves or Men in the Outer Lands. And this is my decree concerning them: to Earendil and to Elwing, and to their sons, shall be given leave each to choose freely to which kindred their fates shall be joined, and under which kindred they shall be judged. - Silmarillion, chapter 24 "Of the voyage of Earendil...
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Thus it was only to one mixed line that the Choice was granted, not to all half-elves by blood. Mithrellas was not of the right line.
It is NOT about being Mixed, nor about blood quantum. The Choice was a special grace granted to Earendil and Elwing for their great sacrifice: for coming to Aman to plead the case of the two kindreds before the Valar.