(inspired by this question)
Certain Hebrew prepositions, such as אל, על, and לפני, are always conjugated in the plural form even with a singular object: thus אליו, עליך, etc.
Others don't: the single-letter prepositions (ל and ב), and some of the longer ones (כמו comes to mind) don't follow this pattern, and are conjugated in the expected forms (בו, כמוה, etc.). I would have expected that at least ל should match אל, since it's a shortened version of it.
Why is this so?