Buying at once seems more favorable for those who have bulk capital and have already made up their mind to invest same in Bitcoin. They don't see any need keeping those funds or employing DCA since the fund is readily available. This is not a bad strategy provided the intentions are to hold for long to be able to realize profits.
Yeah you have a point, for those who have enough funds that cannot be affected if using a bigger amount to buy Bitcoin can actually buy a higher amount of Bitcoin at a time and however DCA method may not be use for these kind of investment because the investor may already have more than enough money that could buy a whole lot of Bitcoin and still have many reserve funds waiting.
So irrespective of how important DCA strategy is, perhaps there are some kind of investment or accumulating patterns that may not require the need to DCA because sometimes due to the kind of funds we may have, we can decide to invest at once instead of buying bit by bit, so I believe that DCA strategy is based more on the people that has a low capital and that's managing to accumulate Bitcoin.