It has economic assumptions baked into it (deflationary, fixed size of money pool, etc). These assumptions, like all economic assumptions built into a currency, will guide and govern people's behavior and use of it. That means that bitcoin is a human-invented system that, if it came into any widespread adoption, would play a role in suggesting or controlling people's behavior, just like any currency.
The difference is, the people whose behavior would be controlled have no say in changing those economic assumptions. The people who could potentially live in a bitcoin-infused world have no voice in how it operates. Bitcoin, in effect, takes a degree of agency from the people, and this is especially problematic if it became widespread enough that you'd be compelled to use it.
Why does that matter? Because people have a right to democratic institutions. People have a right to vote on or have a say in matters that govern their lives. Bitcoin, as it exists now, takes that away from people.
And, to bring up a couple of things people have said to be me here before, the fact that Bitcoin can be changed (i.e. by writing new code and persuading 100% of the community to adopt it) does not make it democratic. Setting the barrier to democratic participation at "learn to code, or pay someone to code, and then change everyone over all at once to your new software" means that that isn't really a democratic system at all.
And to bring up the point that currency shouldn't be democratically decided, well, no. People have a right to govern their affairs and live in a society responsive to their needs. Moving in the direction of taking some of that agency away is moving in the wrong direction.
I'm not opposed to digital currency in general, but I am opposed to one that operates in a way that I have no democratic recourse if I don't like how it runs.