The mitzva of shofar doesn't push off chilul yom tov because yom tov is a positive and negative commandment. The Ran points out that the gemara does not have to say that yom tov is a positive and negative, there is another technical issue in that the aseh and lo ta'aseh does not happen at the same time and therefore yom tov cannot be pushed off for the sake of shofar. Furthermore the Ran points out that the gemara is making an assumption that chazal made chizuk l'divreihem to be like a d'oraysa.
Although the Ran makes these 2 points independently of one another, to me it seems that the second point somewhat answers the first. The reason why the gemara picks the aspect of yom tov being positive and negative, is that it wants to explain that yom tov is very severe and that is why even a rabbinic violation of yom tov is not transgressed for the sake of a mitzvah of shofar. If the gemara would have picked the technical issue of "b'idna", that is just a technicality in aseh docheh lo t'aseh and would not justify in any way why an issur derabonon cannot be done.
Btw, the magen avrohom (also korban nesanel) answer the question and create a yesod that you don't need b'idna for an aseh to be motzi a rabbim (see chayei adam 68).
Although the Ran makes these 2 points independently of one another, to me it seems that the second point somewhat answers the first. The reason why the gemara picks the aspect of yom tov being positive and negative, is that it wants to explain that yom tov is very severe and that is why even a rabbinic violation of yom tov is not transgressed for the sake of a mitzvah of shofar. If the gemara would have picked the technical issue of "b'idna", that is just a technicality in aseh docheh lo t'aseh and would not justify in any way why an issur derabonon cannot be done.
Btw, the magen avrohom (also korban nesanel) answer the question and create a yesod that you don't need b'idna for an aseh to be motzi a rabbim (see chayei adam 68).
