Well there’s no reason you can’t do both. Buy at constant intervals and also top up at large dips.
The reason it doesn’t work for swings is you cant blindly buy at intervals and necessarily win, it’s all about getting the entry and exit.
With DCA you could be very lucky and catch dips but it’ll be usually buying all over the price action including all the spikes.
On a long term strategy it doesn’t matter as much if it did but swings are usually days to weeks, sometimes I’ll play a bull run on a stock I think is being pumped but still that’s only a month or two (unless you get caught holding the bag)
DCA you’ll do say 12 deposits over the year.
If you swing trade a stock you’ll be watching the action like a hawk.
It’s a different set of skills to swing, it’s higher risk in essentially timing the market. It requires waiting for signals in chart analysis combined with extensive DD on upcoming news and plans. You’ll also want to be following the stock action closely to see how it acts, also looking at key support and resistance points you need for it to break. You also need to watch when the run is ending, and plan an exit and stick to it.
Easier swings are usually after earnings dips or IPOs, or RNS or very bad news which isn’t terminal to the stock and will bounce.
What I’ve found it definitely helps to swing trades you don’t mind if you cock up and get caught. 
Also that on the dump let it fully bleed out before diving back in. And avoid the FOMO and temptation to jump in chasing, if you see a stock being pumped by retail on social it’s usually a good sign to exit.