1. The innovations which includes the problems being addressed by bitcoins at very first time in human history.
2. Characteristics of bitcoin as a speculative asset.
3. Technically compatible in these day's Internet and smart phone's era.
4. It is strong community.

I guess this list may go endless but I tried to mention only the few which are more vital than other factors. I believe bitcoin will retain its place for sure in coming decades as well which may last up to centuries too.