As others have posted in here, it is probably down to logistics, convenience, and the possibility of including something from the Prequels.

For me Alec Guinness is Kenobi, the one and only.

Ewan McGregor, James Arnold Taylor, Sam Vincent all do good jobs at playing Kenobi, with Ewan doing particularly well as he could with the material given to him in the Prequels, but they are simply substitutes for the real thing.

In the Sequels, with the “big 3” from the OT appearing in it, it really should have been more of Alec channelled as the voice, be that through an impersonator, his vast previous body of work, or a voice AI tool. That’s obviously just my opinion, though.