If they used explosives large enough to possibly seal the well it would damage other wells and platforms that are in the immediate vicinity thereby causing other leaks. There are literally hundreds of wells within a relatively short distance of one another in that area. An explosion would certainly remove any chance of getting that emergency valve closed and could even make the leak worse. If an expolsion could safely seal the well BP could later drill back into it from the sides. Therefore if saving their well is what they are trying to do then the best way to do that would be to seal it ASAP and drill back into later. Considering what this disaster could could wind up costing BP I would think they are just as anxious as anyone is to stop the leak.