Why does less erosion & more deposition take place on the outside bank of a meander?

That is because the outside bank shallower and have a lower discharge.

Meander can usually be found at the lower bank, where the stream has a higher discharge, has a deeper bank and is smoother.

As the discharge went through the meander, much energy is focused on the inside bank, making the curl in the river and the inside bank is deeper.

While the outside bank is not the focus of energy and is shallower comparing to inside bank, which allows deposition to take place.