it has been functional for tens of thousands if not millions of people for nearly a decade
and it has been subjected to many organized attacks that have all failed or resulted in improvements to the btc code(http://www.autoadmit.com/thread.php?thread_id=3847790&forum_id=7#35068629)
2. Institutional investment/interest
3. Tons and tons of btc whales and miners who've made a killing and take immense pride in keeping it on top(http://www.autoadmit.com/thread.php?thread_id=3847790&forum_id=7#35069007)