Because the excess titrated will be a small number if 10 mL or less and the error in reading the buret for small values is high. For example, you can red a buret to about 0.02 at the best. If you make an error of 0.02 in 1 mL that is an error of (0.02/1.0)*100 = 2% or 20 parts per thousand which I abbrev ppt) But if you make an error of 0.02 in 10 mL it is 0.2% (2ppt)and in 40 mL it is (0.02/40)*100 = 0.05% (0.5 ppt). Since we like to do AT LEAST no worse than 2 ppt, 10 mL is about the minimum value to use. And more is a little better. If a small amount is expected, you CAN use a 10 mL buret (or a 5.0 mL buret) which is calibrated with more precision than the 50 mL burets and that reduces the error somewhat.
