That’s because it was a broad field for broad intelligence enhancements. If you think of it like this, Manhattan Method is my interpretation of a starter pack and can be seen as a replacements.
Now, practically speaking, we have entire fields that focus on specific aspects which Permanent Brain enhancement targeted. For example, the language part only targeted the broca and wernicke areas, A key to Babel goes into much much more detail to make you a linguistic polyglot genius. Manhattan’s mathematical madness will focus more on math. Drops of memory goes way deeper into memory than permanent brain enhancement’s hypocampus stimulation.
They have more content and are more focused.