- avoids the problem of the legs getting into the field of view.
- Is usually easier to move up and down. If you raise a center column to do that, then the legs interfere even more
- Besides getting into the field of view, leg angle locks don't allow precise height control, a further twist on the above two items
- The footprint of the splayed legs of a tripod can require a lot of table space, depending on the leg angle lock used.
- at least in the normal leg lock position, the tripod is not its most stable when the head is tilted down 90 degrees, with the camera hanging very off center.
- the multiple light arrangements of some copy stands can make life much easier, especially when lighting large paper documents. Trying to get even light spread over a document, especially one larger than 8x10 or so, can be very frustrating. Smaller subjects are easier to deal with.
One solution to the height problem is something like the Kirk or RRS long rails, allowing a lot of vertical position adjustments. But those bars alone can cost more than a good used copy stand.
I may have missed a few; the above is just off the top of my head, thinking about trying this with a tripod.
If you just have occasional needs, a tripod may work, and I have tried it just in the interest of science even though I have a fairly decent copy stand.
Once you have tried a tripod, you will quickly find out if the points above are a problem for you with your specific shooting subjects, subject size and distance requirements. Then you know the solution.