because Insulin syringes have longer needles and are meant to be put deeper into your body than TB syringes, therefore making it easier for someone with hard to hit veins (actually, just easier in general regardless of your vein status) to shoot up properly than a TB syringe
reason 2 (although this one is pretty minute as people arent stupid): it makes a little more sense to be carrying a syringe used to inject Insulin than one used to test for Tuberculosis (a disease which most civilized people are inoculated for at an early age). Needles are needles regardless, so your mileage may vary with this reason
reason 3: because they both work and Insulin syringes are easier to obtain. There's a reason they call them "works". Id shoot up with a computer thermal paste applicator if it did the job and was somehow possible. I just want it to work
if that clears it up