It probably has a lot to do with his reputation. Much like the shinobi villages formed an alliance to defeat Madara, or rather a person that they thought was Madara, a lot of how people perceive Hashirama is based on his reputation.
That being said, many people seem to think that there is something special about his DNA. Orochimaru attempted to clone Hashirama, presumably to eventually take over that body. His Wood Style is also one of only a handful of jutsus capable of suppressing a tailed beast; and is only one of two that can do so with a single person. These are all traits unique to Hashirama, which make other Senju less desirable.
