With more channels to reproduce, Dolby Atmos may use a lower sampling rate so they can fit the data on the disc, hence the lower sampling rate. I'm not up on Atmos, so I don't know if there are additional technical limitations. The technical explanation of it all would take a while to figure out and report back here, since there are now so many variations on Dolby and their competitor DTS that involve multichannel audio and different flavors of lossy vs. lossless music reproduction.
Just for clarity, though, when something says "96/24" or similar, they're referring to the sampling rate (96kHz--the data is sampled 96,000 times per second) and bit depth (24 bits per sample, AKA the number of "stairstep" levels in volume/amplitude that can be detected). Bitrate is a calculation: sampling rate x bit depth x number of channels. I remember there was a bitrate limitation on DVD players to where a 24-bit/96kHz/6 channel could be reproduced lossless, but not 24-bit/192kHz/6-channel. (And on top of it, DVD-Audio used MLP--Meridian Lossless Packing, basically a type of data compression--so that the DVD's hardware could keep those surround programs within the bitrate the player hardware could handle.)
Short version--it gets really complicated, really fast.
Just for clarity, though, when something says "96/24" or similar, they're referring to the sampling rate (96kHz--the data is sampled 96,000 times per second) and bit depth (24 bits per sample, AKA the number of "stairstep" levels in volume/amplitude that can be detected). Bitrate is a calculation: sampling rate x bit depth x number of channels. I remember there was a bitrate limitation on DVD players to where a 24-bit/96kHz/6 channel could be reproduced lossless, but not 24-bit/192kHz/6-channel. (And on top of it, DVD-Audio used MLP--Meridian Lossless Packing, basically a type of data compression--so that the DVD's hardware could keep those surround programs within the bitrate the player hardware could handle.)
Short version--it gets really complicated, really fast.