R. Shneur Zalman of Liadi explains (Likkutei Torah, Vayikra 1a ff - partially adapted here) that this "calling" was in order to elevate Moshe to the point where he'd be able to enter the Mishkan (since it was covered by the cloud of Hashem's glory, as described at the end of Shemos - RSZ identifies this "cloud" as a revelation of G-dliness that surpasses all human understanding).
Any name of Hashem that might be used here, then, would delimit that "calling" as coming from some expression of G-dliness represented by that name. By phrasing it, so to speak, anonymously, we understand that this "call," and the love of Hashem for Moshe that it expressed, comes from Hashem's essence.