Well, somebody from the consulate will probably never tell you the reason so all you're left with is "heard." I just have guesses. First, I guess the COs are given a stack of visa applications every morning to process through. Then, I guess that said CO doesn't really want to or have time to go through a stack of asset documents such as bank account statements, stock holdings, etc (in addition to all of the paperwork already inherent in the visa process). Finally, I guess that a CO who may have negative consequences for making a "wrong decision," may feel a bit more confident knowing that another 3rd party (a joint sponsor) is also willing to put their neck out for said visa applicant.
These are just a few guesses and I doubt they even come close to encompassing all the possible guesses out there but, in my mind, these seem most logical. My recommendation is, if you are using assets only, to try it and see but also understand that they may require a joint sponsor so it's better to have one lined up and it will add more time to the visa process.