A couple of reasons, but they aren't show stoppers:
1) the thermocline; water below that line is virtually useless to fish in summer..if you plan to aerate, then ok.
2) cost; it simply costs more to go deeper, other things being equal. I would rather have more surface area and less depth , because that generally is better for fishing.
If you plan to maintain your water level with a well anyway, it seems your money would be better spent on maximizing surface area as opposed to depth. Depends on your objectives for the pond. Hope this has helped.
