Not many people were mining with high-tech mining machines when Bitcoin was released (2009, if I'm not mistaken).
Thus, bitcoin was vulnerable to a 51% attack by a big company or an individual with huge resources.
But was there any such attack on Bitcoin around that time?
There are times when some big pools have enough cumulative hashrate if they combine together but they did not do it. I have never read any report about past attacks and surely I can miss something.
My thinking is if in history of Bitcoin, there has ever been such attacks, Bitcoin would have never been continued to grow more like we are witnessing. Any successful attack will damage the reputation of Bitcoin blockchain and investor belief will vanish. Impossible to reclaim it and Bitcoin would have very cheap price.