What makes a game unmoddable?

[Avante]

Some retro games are moddable by design (have built-in map editors), others were disassembled long time ago and their level formats decoded, and custom editors created, others just have their source code published, which makes it easier to decode the format of resources inside the game executable.
But what makes a game completely unmoddable for years?

Examples: The Lemmings; The Lost Vikings; F-19 Stealth Fighter

[tahishae]

No game is unmoddable. However there's a few reasons the barrier-to-entry might be higher for older games:

• Lack of interest. Most people are interested in playing newer games. Older games, especially niche ones, have a much smaller audience.
• Specialized hardware knowledge. Most modern games are playable on x86 (Intel) architecture. However most older consoles had their own CPU architectures. Modding games for these consoles requires knowledge specific to that console. This also means modders may need to write their own reverse-engineering tools.
• Natively compiled code. Nearly all older games were compiled to native machine code, which tends to be a lot of work to reverse engineer. Many (but by no means all) modern games are written in languages like C# or Java, which are compiled to intermediate languages; or have their logic written in a scripting language like Lua. Both cases make the game much easier to reverse engineer.
• Modern gaming engines. Most older games used game engines that were specific to that game, or at best, that development studio. Most modern games use one of only a handful of engines (usually Unreal or Unity). This allows modders of different games to share knowledge and tools with each other, again lowering the barrier-to-entry.