Sega had a policy before Winter CES 1992 where they would mail out prototype versions of their games to game magazines to preview, which was different from many other game companies at the time which would only show case their early build of games at tradeshows. This was done so that Sega's top games would get much more magazine coverage than normal, a tactic they used to help them rise to prominence and overtake nintendo in global sales in the early part of the 90's, as they began with less than 5% of the total market and were a non-factor coming out of SMS era. By sending their prototype games to magazines, they could take pictures directly in their office and play and write about the games at length.
That backfired when the build of Sonic 2 known as the Simon Wai beta managed to leak into china, where pirated copies of it hit the market before Sonic 2 was even released. This caused Sega to change their practice, which is why games after 1992 are much rarer to get a hold of. After this change, Sega would only mail out tapes of their games to magazines, and only let the press play their games at tradeshows or at events at Sega of America. This is part of the reason it was so very hard to get our hands on Sonic Xtreme, as it just was never in many people's hands like prior games.
That hasn't stopped them from coming, though. Another factor in Sonic games releasing more often is that, after Sonic 1, Sonic the Hedgehog game development moved to the United States. Yuji Naka actually quit Sega of Japan over a dispute after Sonic 1 and was going to leave the company totally, but Sega of America swooped in and rehired him. As a result, Sonic Team was disbanded until they reformed to make NiGHTS into Dreams, and Sonic 2, Sonic 3, Sonic & Knuckles, and Sonic Spinball were all made in San Fransisco at Sega of America under the studio name STI (Sega technical institute). Because of this, it's much easier to find people who still have their debug hardware, or to find development computers in general, as the west has much better archival practices than the east. Western game development evolved out of University comp sci courses, where eastern game development was basically cowboy coding. As such, western development tends to be better at archival. For example, in Japan, it's uncommon to find old computers *at all*. In some areas of japan, it was even en vogue to recycle your computer entirely when your company would upgrade, so they outright don't exist anymore as much.
On top of all this, sometimes sonic stuff it outright found in the completed game -- this is something that can happen just as often with other game franchises, but doesn't, because the Sonic Hacking scene is much more organized and driven than most other hacking scenes. The Sonic hacking scene goes back to 1997 and has been one of the most active retro console demoscenes over the last 20 years. Lots of people taking apart the games, means more eyes looking for lost content. For example, the prototype stuff from Sonic Adventure, like the lost prototype version of Windy Valley, is actually found from the Gamecube release of Sonic Adventure DX, where they used a slightly older version of Sonic Adventure as the base to port from, and didn't totally remove the unused content. Another example is the Java Mobile Phone versions of Sonic 1 released back around 2006, it was built from a slightly earlier version of Sonic 1's source code that featured lost graphics, like the UFOs in Marble Zone, and more importantly, they forgot to strip the debug symbols out during assembly and compilation, so we could take them to apply them to the machine disassembly to arrive back at the original source code, with the original labels and everything.
