The problem was that Andrew Johnson started to reverse this in January 1866.
Of course.
Sherman had to get rid of the thousands of slaves who had followed his army as it marched to the sea, and this arrangement was the quickest way of doing so. He was undoubtedly aware that he had no authority to give them any title to the land in question, but it got them out of his hair, which was all that mattered.
Sherman was like that. When he needed to do something right away, he did it first and let the longer run take care of itself. In like fashion, he signed peace terms with JE Johnston in full knowledge that he was grossly exceeding his authority. But he also knew for virtual certain that once Johnston's army was surrendered it was likely to stay surrendered, and would almost certainly never resume hostilities. So he signed the "treaty" and left Washington to do what it would.
In any case the order applied only to a small area which had been in Union hands for several years, so that its current population was overwhelmingly Black. Any attempt to do the same thing anywhere with a significant white population would just have led to the deaths of any Freedmen rash enough to take it seriously, even had the Federal government ever contemplated doing it, which was never even remotely likely.
With the war coming rapidly to an end, Priority #1 was to get the South back onto a peace footing, and reconcile it to the restored Union asap. This clearly implied that all Rebs, save perhaps a few top-rankers, would need to be pardoned, whereupon, of course they would reclaim their property. Had someone other than Johnson been POTUS, this might have taken a few months longer to happen, but happen it would.
Incidentally, without Andrew Johnson would the Freedmen even have got the vote, let alone land? Congress was wary of the suffrage issue, not mentioning it in the 14th Amendment, and postponing the 15th until the 1868 election was safely out of the way. They adopted Freedmen Suffrage in 1867 not from choice but because it was the only way of breaking the impasse which had developed between themselves and the POTUS. Had Johnson been more co-operative, and twisted the South's arm until it ratified the 14A, they would in all probability have dropped the matter with a sigh of relief. If they are that hesitant even over granting the
vote, there isn't the remotest chance of their embracing something even more controversial.