Why does current flow from positive to negative terminal of the circuit when electrons move from negative to positive of the circuit?

As a convention, the electric current is assumed to be in the direction of movement of positive charges.
You must be wondering that there were no positive charges but electron! you are right. If you are considering metallic conductors than the carriers are electron only. The +ve terminal attracts electron and -ve terminal repels it , thus an electron (-ve charge) moves from -ve to +ve terminal.
But you should understand that excess of -ve charge is same as deficiency of +ve charge. So saying that -ve charge flowing from -ve terminal to +ve terminal is same as saying +ve charge flowing from +ve to -ve terminal. Its like saying that taking a loan of 100rupees is same as depositing 100 rupees into bank.

Now you must be wondeing that why do they set this convention only. Well, I am not very sure about this but I had read it somewhere in +1 level books that while experimenting with electric fields and charges, the +ve charge was somehow easily available. I am inferring it from the fact that in all those experiments, +ve charge was taken as the test charge. This is clearly evident from the description of direction of electric field in our books - "The direction of the electric field is always directed in the direction that a positive test charge would be pushed or pulled if placed in the space surrounding the source charge." So they took direction of current to be same as direction of movement of +ve charge.

However, while solving circuits by Kirchoff's laws, you can assume either of directions for current, and if you pay proper attention to signs, you will get the same results.