The answer is culture.
That is one possible answer, yes.
But it is first and foremost a possible answer to the question that should be asked and answered anyway: why is it a low-magic setting (in other words, why is magic rare) ?
- is it dangerous, and how ?
- is it unpopular, and why ?
- is not not worth the time spent studying it, because there are more powerful or efficient options ?
- is it very difficult to access ?
Depending on then answer(s), the consequences on the game system will be different: the first answer (dangerous) would probably require an added mechanism to make things dangerous when casting a spell (this is MERP's take on magic, in a way - the danger being the risk of being noticed by Sauron or its servants); the second answer would require a very significant social stigma attached to magic use and possibly everyday ostracism and harrassment from "normal" people (some kind of Ars Magic "gift" side effects); the third would require both a reduction in magical power and an increase in DP cost; the fourth would require both modifications on talent requirements (a high-cost "remote master" talent, plus favors each time the character wants to be taught something new, plus any hoop the GM can think of).
Note that some answers may be combined - for instance, answer #1 might trigger answer #2 as a side effect... which might itself trigger answer #4.
But you have to explain why magic is rare in non-technical terms first. The rationale is the logic behind the technical modifications you will make.
Just an idea off the top of my head: magic is rare because, if you have the gift of magic, you essentially become a slave to someone else - this is the only way you would survive. Magic is not rare, because it is useful. Slaves are actually bred (like horses) in the hope the magical gifts will breed true. But the rule is: if you've got magic, you're a slave and someone else has the power of life and death over you.
Of course, some would try to break their chains. Some would succeed. But they are always hunted down mercilessly, because they are a danger to the whole social construct, so no one gives them shelter or helps them.
Because if magic users are not slaves, then they would most likely be overlords. And no one wants to live as a slave to a wizard.
Would you be a slave ?
Now try to build something with this
(there are many questions that are not answered, of course - how are magically capable people detected, how can magic use be perceived by non-users, is there some kind of inquisition force hunting down the escaped slaves, how is slavery enforced and what are the rules for it, are there people trying to cheat - learning magic without anyone knowing...)