The people who are dying and/or those are being hospitalized with serious illness are those who haven't been vaxxed at all. Almost no one who is vaxxed is dying, but the there are so many more people getting omnicron, because it's so contagious, the sheer number of infections is overwhelming the hospitals. Prior surges had up to 200,000 new cases per day. This one has 750,000 new cases a day. Even if half the number of people get really sick with this one, there are so many more of them, it's catastrophic to the system.
Added to which, there are some areas of the country where barely over 50% of the people been vaxxed, giving the virus so many more unprotected opportunities. Few states have reached the 70% threshold needed for "herd immunity".
In summary, Delta attacked the lower respiratory system - lungs, heart, circulatory systems, and Omnicron the upper respiratory tract - nose, throat sinus. But while it is less deadly, it a lot more contagious. The number of cases have tripled. And unvaxxed people are at a much higher risk of both hospitalization and death. Areas with low vaccination rates, are seeing higher hospitalization and death rates. It's a bit of how many more people are getting this version of the virus, even though it's not as deadly.