More than likely due to what WotC will let them sell as those are all owned by them. But probably would need someone from Paizo to say for sure.
My guess is that there is some issue with the advertisements in the old issues that require their removal. This was a problem that TSR had when they released the old Dragon magazines on CD ROM back in the day. I even went so far as to compare the issue I just purchased, Dungeon 139, to a scanned version on Archive.org. Sure enough, several advertisements had been removed from the version that I just downloaded. I hope if this is the case that Paizo has a plan to update these old issues and make them available for PDF purchase again. I know that's a lot of effort on their part but there is some really excellent content in those old issues.
Just to be clear Paizo can not do that. Wizards of the Coast would have to.
If memory serves the savage tide issues were never released as PDFS (Except maybe the first one?) because the deal ended before it came to that point.
Yep. Paizo converted what they could before the license agreement expired (and were allowed to continue distributing those that theyd got to) but they didnt get to the end of the run.
The last few Dungeon magazines were never available as pdfs, sadly.
That's my recollection as well.
The only Savage Tide Dungeon magazine pdf that was ever sold by Paizo was #139, which still looks to be available for sale.
Again, my understanding is this was a Wizards decision, NOT a Paizo one.
Yes, this was WotC terminating the licensing agreement, which led to Paizo's inability to make PDFs of the remaining Dragon/Dungeon issues.