The Gemara relates that R’ Shimon bar Yochai would dress himself for prayer.  Tosfos (Shabbos 11a) points out that this must be referring to the recital of Krias Shema since they were exempt from tefillah.  The Gemara in Brachos (24b) says that merely covering oneself is sufficient for Shema, unlike tefillah which requires proper dress.  If so why was being covered by sand not sufficient?