They had so much power and influence and squandered all of it. They made the same mistake as Labour c. 2005 and Lib Dems c. 2014, thinking they’d be in power forever. But power is fleeting. You must make the most of what you have at the time, to further your own goals. They never did. Looking at the numbers, one of their biggest fuck-ups was the IVs, where they voted with the government. Had they switched, to provide the UK with more options, Customs Union would have passed with a majority of 17 (thereby potentially creating a path to eradicating the Irish sea border), public vote on the final deal would have passed with a majority of 12 (thereby—if it managed to get through—almost certainly reversing Brexit entirely), and Common Market 2.0 would have only lost by a single vote, thereby arguably becoming viable for consideration as well.
A united Ireland now looks inevitable. How the DUP exists within that scenario, who knows?