(a) RASHI (3b, DH Afilu) explains that the Rabanan decreed that such an item
is not Batel because of the following logic: since the item will be
permissible if one eats it at a different time (after Yom Tov), one should
not eat it on Yom Tov, relying on Bitul or on the principle of "Safek
d'Rabanan l'Kula."
Rashi's reasoning might be related to the Machlokes Rishonim concerning one
piece of Isur that fell into two pieces of Heter, and then the mixture was
cooked. In such a case, does the Isur make the entire mixture forbidden,
because once it has been cooked, all of the permitted items certainly
absorbed some of the forbidden item, or does the mixture remain Mutar since,
when the Isur initially fell in, it became Batel in the majority of Heter?
The RASHBA in Chulin (100a) says that the mixture in such a case is Asur.
The ROSH in Chulin (7:37) says that once the Isur was Batel, there is no
longer any element of Isur in the mixture and the mixture cannot become Asur
when cooked.
Rashi might hold like the Rashba, because he says that even though the item
which is Asur is mixed with enough items of Heter to be Batel, since the
item is a "Davar sh'Yesh Lo Matirin" it is better to wait until it becomes
Mutar. It seems that Rashi holds that even when an item of Isur fell into
enough items to be Mevatel it, there is still an element of Isur in the
mixture.
(b) The RAN (Nedarim 52a) explains that the way Bitul works when an item of
Isur becomes mixed with items of Heter is that when opposites combine, their
contrast serves to be Mevatel each other (whichever one is the majority is
Mevatel the one which is the minority). However, when like items combine,
they cannot be Mevatel one another.
Normally, when something that is Asur becomes mixed with Heter, one is
Mevatel the other. Even though the two items are the same type of food, they
contrast because one is Asur and one is Mutar; they are opposites. However,
if something is Asur now and will become Mutar later, it is not Batel
because there is not enough opposition; it is as if the item of Isur which
will later become Mutar is Mutar right now. This is why the stringency of a
"Davar sh'Yesh Lo Matirin" exists only in a case of a mixture of Min b'Mino
-- two of the same types of food (such as an egg that is Asur which became
mixed with many eggs that are Mutar). In a case of a mixture of Min
b'sh'Eino Mino (two different types of foods), even if the Isur will later
become Mutar, there is still enough opposition to be Mevatel it, due to the
different nature of the objects themselves. In a mixture of Min b'Mino, the
only opposition is the fact that one item is Asur and the others are Mutar.
(See also Teshuvos ha'Ran #51.)
According to Rashi's reasoning, why does the stringency of "Davar sh'Yesh Lo
Matirin" not apply to a mixture of Min b'sh'Eino Mino? Perhaps it is because
in a case of Min b'Mino, the taste remains the same, and thus one is
actually tasting the taste of Isur. In a case of Min b'sh'Eino Mino, the
taste is that of the Heter, and therefore the taste of the Isur is Batel and
there is less reason to prohibit eating the mixture in order to eat it at a
later time without relying on Bitul. (See SHACH YD 102:5, who cites the
Rema's answer to this question, who may mean to say what we have stated.)
The Acharonim (TESHUVOS REBBI AKIVA EIGER 1:189) point out that the Ran's
explanation will not suffice to explain why the stringency of "Davar sh'Yesh
Lo Matirin" applies to a Safek d'Rabanan. A Safek d'Rabanan is an item which
*might* be Asur mid'Rabanan, and does not involve any mixture of items.
Since there is no question of Bitul (which applies, of course, only in a
case of a mixture), there is no opposition to be Mevatel the Isur. It must
be that the Ran also agrees to the logic expressed by Rashi. Why, then, does
he add his own explanation for the logic of "Davar sh'Yesh Lo Matirin?
Perhaps because he holds that Rashi's logic will only suffice to explain why
we are more stringent with a "Davar sh'Yesh Lo Matirin when it comes to
Safek d'Rabanan. By Bitul, though, there is no point in waiting for the Isur
to become Mutar since the object that is Batel is no longer considered an
element of Isur, as we mentioned above.