I don't think a lot of it is
actually taxes but that "blame it on the IRS" has been a PR spin win for Hollywood. (Some small amount of it is taxes, which muddies the waters just enough for plausible deniability and PR spin.)
I think some of it is plain old opportunity cost and "vaulting". Streaming a classic on your own streaming network doesn't generate money like selling DVDs/Blu-Rays or another streaming network paying you to license it.
I think a lot of it is probably residuals owed to writers and actors and other staff. Many of the shows being vaulted right now are the sorts of "steady trickle" viewership shows with presumably expensive residuals owed to current creative talent: enough people are watching that the companies see the residuals they are paying on it, but not enough people that they like how much they are paying for it.
Maybe some of that is also how much of the vaulted shows are tied to writers and actors actively striking. It seems a bit unlikely of a coincidence in timing how many of these shows disappearing are happening at various stages in the strike. I would hope the National Labor Relations Board is paying close enough attention and taking notes to try to find out just how much is "coincidence" and how much is retaliatory income cuts.