why is the impact factor of mathematical journals "often" lower than the impact factor of journals in other disciplines
There are a lot of different factors, and I know of no reliable way to determine which is the best explanation.  For example, one theory is that mathematics simply has less impact (in the non-technical sense) than most scientific fields.  I don't believe this, but it's hard to give a principled refutation.  The whole subject strikes me as a little silly, with lots of opinions and numbers with no clear meaning.

One important factor is clearly that mathematicians write fewer, often longer papers than most scientists.  Another is the two-year cycle mentioned by walkmanyi (citations after two years do not count for impact factor, which is incompatible with both the time lag in mathematics publication and the time required to carry out research in mathematics in the first place).

Another factor is the size of the field.  The highest impact factors should occur in an enormous field with some incredibly important research and also a ton of less important papers that cite the great ones.  Mathematics is just not that large a field (compared with biology or medicine, certainly), and it furthermore fragments into a lot of subfields it's difficult to move between.  When someone makes an amazing discovery in algebraic geometry, you aren't going to get a flood of mathematicians from other areas rushing in to take the next steps, because algebraic geometry requires a lot of background.  I don't think that's a bad thing for mathematics as a whole (the things the would-be algebraic geometers are doing instead are probably as valuable as following the latest trends would be), but it cuts down on the opportunities for amassing citations quickly.

Ultimately, I doubt there's any conclusive or satisfying way to determine how much of a role each of these factors plays.

For some published commentary on impact factors in mathematics, see Nefarious Numbers by Arnold and Fowler and Impact Factor and How it Relates to Quality of Journals by Milman.  The first paper focuses on the flaws of impact factors and their abuse/manipulation, while the second explains how impact factor calculations relate to mathematical publication practices (and some of the incentives for journal editors).  Neither directly answers the question here, but they both shed some light on it.