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?
Why attack something that did not yet have any traction? After the release in 2009 Bitcoin had many vulnerabilities comparing to which 51% attack did not sound all that scary. For example, that one time, in the block 74638, 92
billion bitcoins came into existence, you can read more about that here:
https://bitcointalk.org/index.php?topic=822.0I've never heard of any actual 51% attack on Bitcoin, but at one point (very briefly) a mining pool called ghash.io went above 50% of the total hashpower, but they had no intention of attacking the network and took steps to discourage individual miners from joining their pool.