Honestly, i have no idea. Any manager with half a brain would see that it's easy money as it would sell a shit load on curiosity alone. And the explanations generally "accepted" by the community frankly fail to 100% convince me, i'll quote Spaghetti's post for convenience:
Spaghetti wrote: A.) Time - They likely had a fire lit under them after the success of the Kickstarter, but a HD remaster takes time and effort if you're going to do it right.
B.) Legal complication - There have been mutterings about legal complications stopping SEGA properly making a Shenmue remaster happen. Some say they're related to the branding tie-ins (Timex, etc), and others that middleware used to make the game no longer have identifiable owners due to insolvency and asset stripping of the original owners. Neither of these have been substantiated by fact, but it could be true.
C.) Legacy - Shenmue was wrongly a scapegoat for the death of the Dreamcast. Like a game of telephone, inaccurate and inflated budgets have been passed on as fact, and the true cause of death of the Dreamcast (bungling all hardware from the Megadrive onwards) was disguised by executives. SEGA have recently gone back on this though, especially since the KS success put significant egg on their face. They're acknowledging that Shenmue III is a big deal, and they know Shenmue sells judging by how much they've been farming it out for merchandising and allowing cameo appearances from Ryo.
D.) Tie-In - They want to ride Shenmue III's hype, and likewise Shenmue III wants to ride the wave of renewed interest a Shenmue collection would bring, but Shenmue III isn't due for another 21 months at the earliest, and they want to be released close to each other for maximum effect.
A) Sure, i agree, but why not at least announce it then and start building up the hype, if you are serious into the project?
B) This is maybe the only reason that somehow could make sense, but i doubt it's because of stuff like timex and coca cola (all they need is to change the textures). Completely wild guess but maybe they lost the source code? i know it happened to Final Fantasy 9 and that's why it still hasn't gotten a single remaster (it's finally coming tho, and i read somewhere that the devs handling it said development was a nightmare).
C) I agree in theory, but i refuse to believe that after that huge KS success Sega's managers can't see that an HD remaster would make easy millions on hype alone. No one can be that dumb.
D) I don't see how postponing the HD remaster could help Shenmue 3, or how releasing close to Shenmue 3 could generate more hype for the remaster than it would already have. If anything, it would be better to release the remaster as soon as possible so that as much people as possible can get into the series.
My best guess is that for some technical problem the remaster would cost more than Sega is willing to spend on it, there's no other logical explanation.