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?
In those years, there was no obvious reason for the attack, why should they attack. There is no such reason now, of course, but since the mass it affected was not that big at first, an attack was not even considered. I know there is no 51% attack yet.
Its widespread use and popularity today are just some of the biggest obstacles to attacks. I don't think there is such a risk for Bitcoin. This is just one of the beautiful aspects of Bitcoin.
At the same time, the cost of such an attack is very high, and I don't think there is any organization that would want to do something like this.