Because grinding away her cranial ridges wouldn't remove the fact that she's half Klingon.
From the start, the writers of Star Trek have reinforced the ethical use of medicine and healthcare (mental and physical), and it's clear and the writers have thought long and hard about the ethical issues that arise and deal with them in a sensitive manner.
A quick internet search resulted in a very interesting document produced by the John Hopkins University Press back in 2001. Although the paper is fairly old, there's no reason to believe that the approach the script makers made to ethical questions is any different since then.
Medical Ethics through the Star Trek Lens
Over the past thirty years, the Star Trek series of movies and television shows have brought the ethical dilemmas of modern science and technology, and the ethical conflicts that arise in a vast, pluralistic universe, to a huge popular audience in a sensitive and accessible way. The “texts” of Star Trek often take the form of philosophic dialogues, in which the freedom offered by the science-fiction genre allows the authors to pose pointed moral questions in succinctly dramatic ways.
Although Torres isn't included in the many examples quoted and cited in this remarkably interesting paper, the attention to medical ethics is clear.
I'd imagine that any Federation surgeon would simply refuse to conduct this procedure and guide B'Elanna toward confronting and dealing with her
parentage in a holistic rather than bone-choppering manner.
Out of universe, I feel it would be extremely damaging to the Star Trek universe to address the multi-racial aspect of this character in such a crude manner, since this is an integral part of this character's background.