Generally, the input referred noise depends mainly on the gm of the input stage. Perhaps using a two stage amplifier allows using more of the current budget in the input pair, thus improving their gm and hence reducing the input referred noise.
If you choose to have most of the current in the input pair and a small fraction in the output stage then you can have low noise performance ( high gm of input and smaller gm of output stage )
Another thing to consider is that two stage OTA has the smallest number of devices compared to single stage OTAs. this reduces the noise contributors and hence also reduces the input referred noise.