Why was Charles Babbage unable to build his machine?

Answer 1

1. He got the idea for a better machine the Analytical Engine, which would have been fully general purpose.
2. When he did not deliver a working Difference Engine as originally promised, Parliament withdrew all his funding.
3. His machinist threatened to take all the tooling and parts he had made to make up for late payments for work already done that Babbage owed him. The machinist quit.

Admittedly on his own Babbage did complete the design of the Analytical Engine and an improved Difference Engine that was much smaller and needed only about one fourth the parts, but with no way to get funding and no machinist he could not very well build them.

Or to oversimplify it, breach of contract caused by Babbage's inability to stop engineering better products and get the one he had into actual production.

Answer 2

Funding.