My guess is they changed it because Tarkin dies when the Death Star explodes, but Vader survives. Thus, there is more “poetic justice” if Tarkin was the one who ordered the destruction of Alderaan.
Tarkin still orders the destruction, Vader is just the one who tells them to release the primary ignition.
He’s the link to the script: https://imsdb.com/scripts/Star-Wars-A-New-Hope.html
Interesting. Yeah, would be interesting to find out what motivated that change. I think Lucas always had plans for Vader to be somewhat sympathetic, but not necessarily Luke’s father initially. I forget the exact details, but originally Vader was an amalgamation of characters from earlier drafts, including a “Sith Knight” character who was shown to have some redeeming qualities, caring about honor, etc. Darth Vader in A New Hope is also shown to still care about some notion of a code of honor, as he chooses to face Obi Wan alone. Also, Vader is kind of unsupportive of the Death Star in general. Maybe Lucas’ intuition told him that Vader should be distanced from the destruction of Alderaan as much as possible.
It’s similar to how, in the novelization or radio drama of A New Hope, the scene where Leia is tortured is described in more horrifying detail than the movie, but the author also strangely tries to make Vader slightly sympathetic by having him order the guards to provide some comfort to Leia after the torture session fails.