Q: All previous attempts to create independent digital currencies were made illegal. Why is Bitcoin still legal ?
A:
They didn't fail because they were made illegal. They failed because they could be made illegal. Satoshi's emphasis in the abstract of his paper is perfect. He solved the double spend problem, which was the last piece in the puzzle that neither Wei Dai nor Nick Szabo nor a teenage Peter Todd could figure out. Solving it removed the need to trust a centralised third party.
Listen to Peter Todd's interview with Peter McCormack on What Bitcoin Did.
Peter Todd thought the fundamental problem was limiting the issuance of new currency. But Satoshi realised the real problem was more broad - how do we make it expensive to update the ledger (which includes but is not limited to issuing new currency). He did this using proof of work as a mechanism to update the blockchain.
Source:
https://www.reddit.com/r/Bitcoin/comments/amzrmu/in_1998_wei_dai_came_up_with_his_ingenious_idea