What was preventing the development of Figma before Wasm?

For developing brand new code, I don't think there's anything fundamentally impossible without Wasm, except SIMD.

Performance. JS can be as fast as wasm, but generally isn't on huge, complex applications. Wasm was designed for things like Unity games, Adobe Photoshop, and Figma - that is why they all use it. Benchmarks on such applications usually show a 2x speedup for wasm, and much faster startup (by avoiding JS tiering).

Also, the ability to recompile existing code to wasm is often important. Unity or Photoshop could, in theory, write a new codebase for the Web, but recompiling their existing applications is much more appealing, and it also reuses all their existing performance work there.

Yet Figma like tools do exist without Wasm.