This is covered specifically in my post, but mainly that zstd is not well supported in older distros we would like to support, bionic for instance does not support zstd compression (last I checked, perhaps the 18.04.4 update now supports it, but it did not originally).
We hope to some day be able to move to zstd for compression, but we want to be sure it is maximally compatible, or barring that we have a fallback mechanism for users who install a snap that their machine can’t decompress they will get an alternative snap file which is just compressed with something they support. That requires even more change than just allowing upload of lzo however.