1. Ease of prototyping - other people have already discussed the technologies. Open hardware has made it easy to build a First Thing.
2. Access to First Thing funding - Indiegogo/KS are not going to fund a company, but they give entrepreneurs access to working capital to build a first run.
2a. Social crowdfunding is an easy place to build an early adopter base to facilitate rapid iteration.
3. China. After making that First Thing, you need access manufacturing. The increasing proliferation of multinational companies devoted to all aspects of manufacturing - sourcing, supply chain management, logistics, fulfillment - mean that while manufacturing is still incredibly complex, it's increasingly manageable.
4. The HW incubator ecosystem. Access to mentorship and a knowledge base. Invaluable. YC may give a founder a network and some buzz, but PCH/Bolt+Dragon/Lemnos give founders access to engineers, tools, and incredible expertise.