Why electricity flows from +ve to -ve terminals?

The electricity does not flow from the positive terminal to the negative terminal, at least not in the technical sense. Electricity flows in the exact opposite direction, that is, from negative terminal to positive terminal. To understand better, one has to be familiar with two types of currents: conventional current and electron current. 

For along time, it was thought that electricity is the flow of positive charge and hence, flows from the positive terminal to...

The electricity does not flow from the positive terminal to the negative terminal, at least not in the technical sense. Electricity flows in the exact opposite direction, that is, from negative terminal to positive terminal. To understand better, one has to be familiar with two types of currents: conventional current and electron current. 

For along time, it was thought that electricity is the flow of positive charge and hence, flows from the positive terminal to the negative terminal. This current is known as the conventional current. Later it was found out that electricity flow is due to the flow of negative charge, that is, electrons. This current flows from the negative terminal to positive terminal and is known as the electron current. And this current is in exact opposite direction to that of conventional current.

Even though we have realized that actual current (electron current) flows from negative to positive terminal, we are still following the older convention and that is why, one often reads that current flow is from positive to negative terminal.

Hope this helps.