Burning SBD is too slow at the available funding rates, even if this proposal was fully funded, only 25K SBD would be burned which won’t make a dent.
By cycling the funds through conversion and market buy action, the same funds can used to reduce SBD through conversion every 3.5 days and have a proper impact on SBD supply reduction.
Remember the STEEM marcap is roughly 7 times larger than the SBD marcap so the impact on Steem should theoretically be 7 times less.
This is all done incrementally so the Steem market should be able to absorb it and it can be throttled/stopped at any time should it affect the Steem price too much.
"Too slow" is a bit subjective but it is certainly the case that paying out SBD from the DAO and then burning them can't possibly do anything to reduce the supply of SBD that is already in circulation, since it uses newly-created SBD from the DAO. All this does, in effect, is reduce the inflation allocated to the DAO.