It's a valid question... And the answer for that question from me is not the ecosystem, as I feel the Julia ecosystem HAS become rather mature and great especially recently, with 0.24 of DataFrames, and the actual reconstruction of WebIO. Of course Flux is always going to be radical too.
The answer to your question is work. I do not necessarily control what I write at work. I love Python either way, I generally tend to explore around a bit -- find things I like, that is kind of how I came to Julia in the first place. The industry currently is overwhelmingly in Python, that's where most aspiring Data Scientists are likely to start. I write Python in my workplace, very few of the people I work with know Julia, and I don't even feel like I would have enough faith in these developers to write Julia code or understand it, or rather write it WELL.
So really it just comes down to where the rest of the world is at, in my mind I already see a very Julian future for Scientific computing, you might notice how overwhelmingly Julia most of the things I do online are, I don't write Python for my open-source hobbyistic programming very often.