I'm sceptical about the record label thing being a reason for the four Ripper albums not being available on streaming services
anywhere. I think it's a lack of effort on the part of Priest as a business.
In different interviews over the years Ripper, KK, Ian and Scott (as recently as late last year) have said they outright don't know who owns the rights. Scott vaguely hinted at the business side of things not being the only reason:
I think the record label went out of business. And with Rob being back in the band, it's kind of probably a delicate situation as far as, 'Hey, we wanna re-release these records.'
Looking at the liner notes for the 50 Heavy Metal Years box, the first two albums explicitly say "℗ 1974/6 Gull Entertainments Ltd" and Rocka Rolla says "Licensed courtesy of Gull Entertainments Ltd/Repertoire Records UK Limited/Entertainment One U.S. LP". When you get to the four Ripper albums, both the phonogram (sound recording copyright) and regular copyright symbols are attributed to Sony. There's no mention of SPV/Steamhammer or their parent Napalm and no mention of CMC or their parent BMG.
My theory is that Sony (Priest's label since Rob came back) owns the rights to those four albums - at least in the UK, but likely internationally, too, given that box was released by Columbia/Epic in the US and Europe.
In 2009 when SPV went into insolvency, Sony bailed them out and acquired a chunk of their catalogue. Given the documentation on the liner notes that the sound recording copyrights to those four albums were attributed solely to Sony (at least in 2021) and not licensed from Napalm or SPV, I think it's safe to say Sony acquired the rights to them in 2009. Priest were one of the higher-profile artists whose work was published on the label and they were one of the names namedropped in the industry announcement when Sony bailed out SPV and acquired some of their catalogue.
So, the label Priest is signed to own the rights to those four albums. Not Priest's holdings company or their own label (Exciter Records), but the company that sells/distributes every other Priest album across the globe, which is the next best thing.
The bottleneck for availability lies in the camp of Judas Priest as a business - Judas Priest Music Limited, which has four people on its board: Glenn, Rob and Ian as directors of the company and Jayne Andrews (their manager/Glenn's partner) as its secretary. Glenn and Ian played on the albums. Glenn wrote and produced them, so he's likely proud of them. Ian didn't even fuckin' know the albums weren't available and he's one of the bosses of the company. Jayne was still their manager during those years, so it's not like she has a personal problem with them. The outlier is Rob, who has said that he's never even listened to the albums and clearly has personal difficulties with that time in his life and Priest continuing without him.
But I don't think it's malice on the part of Rob or anyone else in the band/company. They released all four albums as part of that boxset and they've released official Jugulator merchandise in the last few years (I imported a shirt from Hot Topic and they had some other Jugulator shit on their official webstore).
I think the reason those four albums aren't widely available is the same root cause of every blunder and misstep they've made over the last several decades - the band, as a business, is run like absolute shit and the people running it (Glenn, Jayne, Rob, Ian) don't know how to operate a business to the scale that a band with such a legacy deserves.
tl;dr - those Ripper albums aren't widely available because Priest is run like shit, not because of malice or label/legal complications