The comparative of sacer (sacra, sacrum) should be *sacrior (-ius), but it is not attested (1, 2), even though its superlative sacerrimus (-a, -um) is attested 7 times (3). Is there a reason why the comparative could not exist, or was it simply a lack of attestation?
Phonologically, there is no obvious reason for *sacrior to not exist: there are a number of attested comparatives with similar forms such as macrior, pulc(h)rior, ācrior, alacrior or integrior, nigrior, pigrior.
I agree with Draconis that the absence of this comparative from the PHI classical corpus can be explained as either accidental or as related to its meaning. It appears to be attested in post-classical Latin sources.
Allen and Greenough specifically point it out as a word whose comparative "is rare or wanting" (§131a).
An argument that the meaning of the word sacer was not compatible with the meaning of the comparative is made in Indo-European Language and Society by Émile Benveniste (1973), Book 6: "Religion", Chapter 1: "The “Sacred”", which includes a discussion of the meaning of Latin sacer compared to sānctus. This is available online at Harvard University's Center for Hellenic Studies.
Here are the quotations I found most relevant to your question (square brackets indicate my alterations):
Sacer is an absolute quality and does not admit of degrees. At the most a supreme state is conceivable; sacerrimus ‘sacred above all else’.
[with Varro, L. L. VIII, 77] we have a grammatical text, which is concerned with the formation of comparatives and superlatives. Varro draws attention to the differences presented in this respect by adjectives which have the same form in the positive. He takes the three adjectives macer, sacer, and tener: the superlatives are the same: macerrimus, sacerrimus, tenerrimus. But he cites only two words in the comparative, macrior and tenerior. If he was not in a position to cite *sacrior (although he quotes sacer and sacerrimus) this is because sacer had no comparative, because the sense of the word did not admit of degrees
Sacer isn't an especially frequent adjective (seven attestations of the superlative isn't that many); "more sacrificial" isn't a phrase that's needed very often, and it was in competition with sanctus and sacrosanctus depending on the time period for the meaning of "sacred".
I would chalk this up to the vagaries of time. There are plenty of perfectly good Latin words that don't appear in the surviving corpus just through random chance, and I suspect this is one of them.