From what others have said (very interesting answers and comments), we could infer coverage percentage is really related to cost, while viewfinder size is mainly related to other factors, i.e.

• people who buy low-end SLRs are most likely to expect a smaller and lighter camera; a bigger wiefinder needs a bigger, heavier body like those needed for professional gear, that starts to take money. But the cost of making just some parts (of the viewfinder) millimeters bigger is not likely to be relevant.

• market segmentation strategies, as cmason pointed out. Coneslayer said cmason's point was weak in that he "compared it to features that really do have a substantial cost of implementation". IMHO it's quite clear that manufacturers differentiate their products with both actually expensive features and things that wouldn't cost them a penny, e.g. some features in firmware: in a low-end SLR you can, say, assign three functions to a button, in a camera which has double the price those functions are maybe ten or nearly any function available in the menu.

  A bigger viewfinder is really useful, but 100% coverage compared to 96% or the like is just one more selling point to please professional and amateurs who, for different reason and with different expectations, still both of them expecting high standards in every part of their gear, spend thousands $/€ on a top level camera body.

From what others have said (very interesting answers and comments!), we could infer coverage percentage is really related to cost, while viewfinder size is mainly related to other factors, i.e.

• people who buy low-end SLRs are most likely to expect a smaller and lighter camera; a bigger viewfinder needs a bigger, heavier body like those needed for professional gear, that starts to take money. But the cost of making just some parts (of the viewfinder) millimeters bigger is not likely to be relevant.

• market segmentation strategies, as cmason pointed out. Coneslayer said cmason's point was weak in that he "compared it to features that really do have a substantial cost of implementation". IMHO it's quite clear that manufacturers differentiate their products with both actually expensive features and things that wouldn't cost them a penny, e.g. some features in firmware: in a low-end SLR you can, say, assign three functions to a button, in a camera which has double the price those functions are maybe ten or nearly any function available in the menu.

  A bigger viewfinder is really useful, but 100% coverage compared to 96% or the like is just one more selling point to please professionals and amateurs who, for different reason and with different expectations, still both of them expecting high standards in every part of their gear, spend thousands $/€ on a top level camera body.

Well, this is not a real answer, just wanted to let out some thoughts I came up with reading other entries, and to respond to some of what others said. Nevertheless I think my post tackles the "why?" part of the question, so it's not entirely off topic.