Who choses where supermarkets are sited?

It does make me wonder.  In Morecambe, we have two Tescos, an Iceland, an Aldi, a Lidl, Co-op, Morrisons, a Sainsburys, an Asda and if we count anything this side of the river as Morecambe, a Booths, Spars, (don't know how many of those,) another Aldi and a large Asda. Not to mention places like Home Bargains and B & M bargains,we also have a busy market, and a fairly busy Sunday market
The newest of these is the Sainsburys, and it doesn't matter what time of day you go in, you can get round easily with no crowds and don't have very long to wait to checkout. I wonder who in their wisdom thought we needed a Sainsburys?  There is one in Lancaster, which is always very busy, but the one in Morecambe never is, they are only a couple of miles apart at the most.
Did the powers that be at Sainsburys get around a table and discuss this?  The more affluent people in the area tend to shop in the Lancaster shop, as they can combine it with a trip into the city. As soon as I knew what they were building I thought to myself that it would never be busy, and sure enough it never is.  Could it be that the prices are much higher than even Morrisons and the Co-op? Nah! why would that put people off. And if you want real quality stuff you would go to Booths, well, I would.
I don't expect an answer, but it puzzles me why when there is already a plenitude of supermarkets from cheap as chips places to high quality places, they went and built another one.