I'm speculating that there are two possible reasons. First, it's common to implement complex CMOS circuits with dynamic rather than static circuitry, because it takes fewer MOS transistors. In such a case, there will be a minimum operating frequency, although you may go below that if you're below the maximum temperature spec. The same should go for the charge on node capacitors leaking away.
Second, some delta sigma modulators have internal integrators that have voltages that increase with time, and a maximum clock period has to be specified to keep them from overflowing.
There are lots of cheap high-resolution low-frequency delta-sigma ICs available, aimed at scales, geo-electronics, process industry and other markets. Why make an awkward use of an audio IC.