Because that's the way it is. See? That is what happens.$${\frac{\left(-{\mathtt{45}}\right)}{\left(-{\mathtt{54}}\right)}} = {\frac{{\mathtt{5}}}{{\mathtt{6}}}} = {\mathtt{0.833\: \!333\: \!333\: \!333\: \!333\: \!3}}$$
Well I can demonstrate with a pattern
4*3 =12
4*2 =8
4*1 =4
4*0 =0
4*-1 =-4
4*-2 =-8
Can you see that the pattern shows that a pos * a neg = a neg ?
Now i will swap it around
-2*4 =-8
-2*3 =-6
-2*2 =-4
-2*1 =-2
-2*0 =0
-2*-1 =+2
-2*-2 =+4
-2*-3 =+6
Can you see how the pattern shows that a neg * a neg = a pos ?
there are other ways to show this as well. One of the other answerers may show you another way. :)