I think, at least here, a major part of it is simply because (right or wrong) they think it will hurt sales of cards. Do you suggest simply trying to lure stores away from sanctioned tournies or what?
Here are some arguments you could use:
Most stores have a very limited selection power cards in stock, if they have any at all. Last time I went to YMG Somerville (a major store in my area), they had one Mox (Emerald) available. It is difficult for a store to hurt sales of cards they don't even stock! By allowing proxies, the store owner actually INCREASES his/her potential of receiving revenue from customers. Would the local store owner rather sell a box of Darksteel (in stock) or lose the $90-$100 sale to, say, a Mana Drain (not in stock) purchase on ebay?
Type 1 tourneys attract segments not currently served by the store. Last time I checked, expanding your customer base is a Good Thing (tm).
Type I tournaments need not be sanctioned. Vintage DCI ratings are insignificant to 99.9% of competitors in Type I tournaments. DCI ratings matter most to the PTQ crowd, for whom Vintage ratings are worthless.