There were several:
* The business argument for electric cars has been there for a long time. It's obvious to anyone who looks at it that there's an efficiency to using electricity instead of gasoline, as evidenced now by Tesla's incredibly low cost-per-mile. That of course doesn't explain Fisker....
* Many of us started believing in Tesla almost immediately after hearing the 3-part business plan: Build a ridiculously fast, desireable sports car (which they did, and brought it to market long before the DOE loan), then build a luxury car, then build a mass market car. I had a sense Tesla would do well with this plan, for the same reason I think Square will do well where others will fail: You need more than a viable destination, you also need a viable stepping stone at every point between zero and profitability. Tesla always had this plan. It was a good one. It's proving itself to be a good one.
* Did I mention they already had built, brought to market, and made real revenue off a car? You can't say that about Fisker.
* Musk had a track record as a successful CEO.