I can't believe the length of this thread.
It's simple.
The last guy to do maintenance and the pilot who did the walk round both failed to do their jobs properly.
It's as simple as that.
Aircraft take off with panels not closed or gear pins still in. It happens. Because individuals fail to do their job.
You can try to push the responsibility wherever you like but the fact remains.
I once left the pitot heaters on and melted the covers onto the probes. It was my fault, nobody else. The procedure was sound. I failed in my duty.
Live with it!