The d20 system which it is based on is distributed under the Open Gaming License, meaning companies can publish and sell their own products for D&D. This resuled in a massive expansion of the product line and a massive user base expansion.
Also, what must be noted is the design of the new handbooks are quite breathtaking, and are in themselves, works of art. IMHO, the most spectacular is the Forgotton Realms book.
The books are also being mass-produced, meaning that the books are significantly cheaper.