Bitcoin is not open source because only a relatively small cross section of Bitcoin users understand the programming.
I use Bitcoin Core for Windows found through the link at
https://bitcoin.org/en/download. The name of the current version file that I download is "bitcoin-0.13.1-win64-setup.exe." The download screen says that the size of the file is 12.5 MB (megabytes). The Windows (zip) of the same program says it is 22.6 MB. I use the smaller of these two.
While I really don't know what the difference is between these downloads, the smaller of the two installed to my "Program Files" directory in my computer, at 41 MB. (The whole blockchain is well over 100 GB (gigabytes).)
Do you know how many lines of code you could write to fill 41 MB of computer hard drive memory? Thousands!
What do all these lines of code do? What do they do to your computer? How do they work with your Internet connection, and what is really being sent over the Internet when you have Bitcoin running? Is there really anybody who knows the whole thing?
The point is, Bitcoin is not really open source, because it is not open to the vast majority of minds that use it. The average person could understand the whole Encyclopedia Britannica easier than he could understand what goes on with the Bitcoin programming in his computer. And probably fewer than 99% of
programmers understand it, to say nothing of lay people.
If bitcoin wasn`t open source then would you pay $XXX amount of dollars or btc to buy that software
that you installed for free?
It`s open source to all the people who understand programming code and it`s free to download and use.
Do you want something more?