Maybe the reason is based on a line in the Silmarillion, a line that was not taken up into the constructed version however [meaning Christopher Tolkien's version published in 1977].
Quote:
'Then Manwe gave judgement and he said: 'To Earendel I remit the ban (...) Now all those who have the blood of mortal Men, in whatever part, great or small, are mortal, unless other doom be granted to them; but in this matter the power of doom is given to me. This is my decree: To Earendel and to Elwing and to their sons shall be given leave each to choose freely under which kindred they shall be judged."
JRRT, The Lost Road, Quenta Silmarillion
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Why to their sons in the first place? My idea is that
if this is true [at least it was when it was written], then mortality would seem to be the default status for the Half-elven, or the natural order of Eru; and this would include that folk with mortal blood cannot be denied a mortal death. And if so Arwen, Elladan and Elrohir would be automatically mortal, having still a measure of mortal blood -- but then they would be, in potential, automatically sundered from Elrond [an 'immortal chooser'] and Celebrian [a natural immortal], even after death.
Elros' children are in a sense, denied immortality while having a measure of immortal blood, but in another sense they simply return to Eru's plan concerning the Half-elven. They are not denied death, nor are automatically sundered from the parent who chose mortality [who would likely wed another mortal I think]
Elrond's children, if automatically 'immortal'
but still having mortal blood, would be automatically denied a mortal death -- and if automatically mortal, would be automatically sundered from their parents. It was Earendil and Elwing that were given special dispensation, but again, why add Elros and Elrond? Well, we can say that they too would be automatically mortal without the same 'other doom' being granted to them.
To me, I think the
potential choice of immortality must include an extension to the children, otherwise [again, at least in light of the above text], having mortal blood, they will have no choice but to die while their parent[s] live[s] on until the end of the World and so on...
... of course even
if all this holds up, one might then ask why it should be [in my opinion] the 'natural order of things' that anyone with mortal blood, in any measure, is automatically mortal,
if no special doom is granted to them.
I don't know
...
... but that's what I think it says in the Silmarillion of the later 1930s anyway.