Because Thomas Gradgrind is a man who lives by rules and the measure of how others adhere to those rules. All the items in this description can be understood both literally and metaphorically. The ruler measures, the scales weigh, the tables tell him instantly how much profit he can expect. Metaphorically, The "rule" measures people, he sizes them ; the scales show him how much those people are worth (or not worth) to him; the multiplication tables show him, again, how much profit he can expect, but in a callous way, when it comes to people.
Here is the passage in full from the beginning of Chapter Two. (You can also follow the link below to the eNotes pages on "Hard Times" for more on this character and other literary elements.)
Thomas Gradgrind, sir. A man of realities. A man of fact and calculations. A man who proceeds upon the principle that two and two are four, and nothing over, and who is not to be talked into allowing for anything over. Thomas Gradgrind, sir -- peremptorily Thomas -- Thomas Gradgrind. With a rule and a pair of scales, and the multiplication table always in his pocket, sir, ready to weigh and measure any parcel of human nature, and tell you exactly what it comes to. It is a mere question of figures, a case simple arithmetic. You might hope to get some other nonsensical belief into the head of George Gradgrind, or Augustus Gradgrind or John Gradgrind, or Joseph Gradgrind (all suppositions, no existent persons), but into the head of Thomas Gradgrind -- no sir!