Hashem would rather overlook the three cardinal sins than overlook Bittul Torah. Why?
The Yeitzer would allow you to fulfill any mitzvah rather than to study Torah: also, why?
First of all, to point out that although a major part of Bittul Torah has to do with the study of Torah, the main point that I refer to here is not the studying of it; rather, mindfulness of the insights themselves (also, the insights have to be holy - from observance - for the following to apply).
Where does human morality come from? We have always known that the Torah is the source; that before the giving of the Torah, Hashem had to somehow take on that responsibility; from then onward, however, we - the Jewish people - are that part of humanity that is responsible for its morality.
Just as the Exodus was brought early to avoid our complete assimilation, and that had we stayed any longer, there would have been no Jewish people to fulfill our role in society; so too, there has to be continuity in our being part of the Torah - now that we're responsible for it - such that, as long as we are still thinking holy Torah insights, we are connected to our sane, moral selves of the Sinai wavelength ( like in the lord of the flies, needing adult involvement for what can be as vulnerable as a hair's breadth of a connection), and humanity can feel the bottom line of morality; otherwise, even an instance of interruption would disorient mankind; which would promptly lose its sense of the reality of morality (whose vulnerability, until then, had been invisible to the user). So, as long as the Jewish people keep up the Torah wavelength, the substance of morality is contended with, and mankind retains its humanity; interrupt it - even for an instant - and the insurmountable doubts humanity would have would be in areas that, until then, they had taken for granted would continue. This is why if some Jew, at any given moment, is the only Jew involved in thinking Torah thoughts, he is called Zaddik Yesod Olam: a moral person who heroically supported all the world - all of human history - which is able to continue, now, because there was no interruption in Torah mindfulness (partially due to him alone), and the reality of human morality can still be 'invisible to the user".
Now, I have established that we are the conscience of mankind, and to that I emphasize that the Sinai wavelength is where we're coming from.
Now, I would like to point out that in human nature, men need a game in a way that the women don't recognize. Women would call us children, but that's because the games we tend to use are often childish; it would have to be a virtuous game for them to even realize that our game instinct might be something other than immaturity. So, what virtuous games can be chosen? There is poverty, hunger and illness troubles in the world that need to be solved; these are recognizably virtuous 'games'. So, humanity should resolve these problems, already, shouldn't they? But what if all of these troubles were eliminated, what would be our game? What would society do? If no virtuous games remain what are the alternatives?
Enter the Torah.
Here we have a pastime that we refer to as Hashem's game!
If we play it, we have an active direction to our game. Here we try strategies as to how to solve those other troubles and what to do should they pop up somewhere. When we find an answer, we can celebrate a breakthrough in virtuous policy!
To answer why Hashem considers Bittul Torah worse than the cardinal sins, consider that the Torah wavelength is moral sanity, and that although transgressions may interfere, this interference still has to contend with the moral lifeline that is our Torah: our conscience; so that although sins interfere with the functioning of our conscience, as long as there are Torah insights shining into human life, we still have the conscience itself with which to fight. If sins are diseases of moral sanity, then Torah - our conscience - is our life and limb; and as long as it continues, so do we.
To answer why the Yeitzer prefers that you do other mitzvahs rather than study Torah; consider first that Torah study is a game that changes your mindset (offering a healthy environment and a pleasurable alternative to a life of moral insanity); every self-respecting idea you learn changes the wavelength you're on. If you do a mitzvah you are trying to light a flame; when it catches you return to your regularly scheduled programming: either nurturing the flame or letting it go out; depending on what that mindset is.
If there is no alternative game, mankind's immature programming will douse that moral flame, anyhow, so the mitzvah alone won't flourish; but if there is, then you may actually let yourself succeed in being a better person; and then what will your Yeitzer be doing? To paraphrase the mishnah: Ki l'kach notzarto; this is the healthy reason you were given a Yeitzer: so that men should be able to play the Torah (study) game.
So, your mindset is what calls the shots; and, for a man, the game he plays determines which mindset is his; so if he plays the Torah game, he is changing his mindset to one that will change his Yeitzer, too: is it then any wonder that his Yeitzer should feel threatened by it?
We should resolve our problems, but we need some way to live in that new beginning; or else, it is the beginning of what?