Why Enlightenment isn't Inevitable.

As for an answer (I will only take the title) to your question;
In order to attempt to answer such a thing, I will use the definition of 'Bodhi' as found on http://en.wikipedia.org/wiki/Bodhi for 'enlightenment': In early Buddhism, bodhi carried a meaning synonymous to nirvana, using only some different metaphors to describe the experience, which implied the extinction of raga (greed), dosa (hate) and moha (delusion). In the later school of Mahayana Buddhism, the status of nirvana was downgraded in some scriptures, coming to refer only to the extinction of greed and hate, implying that delusion was still present in one who attained nirvana, and that one needed to attain bodhi to eradicate delusion ^[1]. Therefore, according to Mahayana Buddhism, the arahant has attained only nirvana, thus still being subject to delusion, while the bodhisattva not only achieves nirvana but full liberation from delusion as well. He thus attains bodhi and becomes a buddha

And so I will define 'Enlightenment' as; 'The extinction of greed, hate and therefore attachement, while achieving full liberation from delusion.
Where concepts like Greed and Hate are personal definition different per person. And 'Liberation of delusion' meaning a form of self awareness that makes on tackle its own weaknesses, and recognizing ones one illusions. And keeping a truthful view of the world around him. Not assuming more then is absolutely necessary.

Now I am able to answer the question:
Enlightenment isn't inevitable at all.
- At first, the extinction of hate and greed are for most people already untouchable concepts. That they will never achieve, for most people do not rise above their physically induced forms of attachment. Then there are many people that will dwell into mental forms of attachment. And an enlightened person will need to know all of these concepts. And must have experienced all such concepts, just so to know what it is he needs to rise above.

- At second; If indeed a person is able to rise above these concepts. And some really do. One still has to overcome delusion. And delusion is a very difficult concept to master, I might even say, that by mastering delusion alone. You will automatically master the first requirements of Nirvana. Out of cheer comprehension of the world around you. It is at first impossible to recognize ones illusion at princeps tempus, it is necessary to reflect upon ones one action, no matter how trivial it might seem. And perhaps reflecting outwards, in order to even get a slight glimmer of self behaviour. And for this one needs to be able to set oneself above emotions, and attachment.

- At third, which is my more personal opinion; I think that in order to do the above. One therefore needs a trustworthy system, or ideology that is able to reflect upon ourselves, and in essence all of nature. That doesn't assume to much, and basically has in it the limitation of knowledge. A system that has built in, all subjectiveness that one could ever encounter, elementarily while observing ones own. And so, when one begins to master his own subjectiveness, there is at least a basis that knows its own subjectiveness. And hence I am a physicist.