Because
the speculative reason is busied chiefly with the necessary things, which cannot be otherwise than they are, its proper conclusions, like the universal principles, contain the truth without fail.
The practical reason, on the other hand, is busied with contingent matters, about which human actions are concerned: and consequently, although there is necessity in the general principles, the more we descend to matters of detail, the more frequently we encounter defects.
Accordingly then in speculative matters truth is the same in all men, both as to principles and as to conclusions: although the truth is not known to all as regards the conclusions, but only as regards the principles which are called common notions. But in matters of action, truth or practical rectitude is not the same for all, as to matters of detail, but only as to the general principles: and where there is the same rectitude in matters of detail, it is not equally known to all.
An example showing how speculative reason can fail in a particular case:
it is right and true for all to act according to reason: and from this principle it follows as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is true for the majority of cases (ut in pluribus): but it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance, if they are claimed for the purpose of fighting against one's country. And this principle will be found to fail the more, according as we descend further into detail, e.g. if one were to say that goods held in trust should be restored with such and such a guarantee, or in such and such a way; because the greater the number of conditions added, the greater the number of ways in which the principle may fail, so that it be not right to restore or not to restore.
—St. Thomas Aquinas, Summa Theologica I-II q. 94 a. 4 ("Whether the natural law is the same in all men"?)