Because cities are responding to the needs of their homeless population pro-actively. Shelters are implementing stricter cleaning requirements, and lessening the amount of people who can stay (to increase distance between residents.) Some cities (like the one I work in) are opening two different, new shelters (and some of these shelters, in some cities, are in hotels): one that is for those who are positive or presumed-positive, awaiting results, and one for those who are medically fragile (and more likely to have a worse reaction to the virus) but have not yet tested positive.
In some cities, homeless folks are dying and people aren't realizing it is from COVID, if they're not being tested (I suspect this is true for more populated cities.)
I am actively involved in the planning of these efforts, so this isn't just armchair speculation on my part.
The death rate rate for those that test positive, across all populations, in my state is staying pretty steady for the past few weeks at about 3%-4%. So much higher than the flu and the 1% that was quoted earlier. I would consider NC a medium-density state. So, in the city where I work, the last point-in-time count of the homeless population in 2020 was 440 people. Based on those numbers, I would expect that about 17 homeless people would die, if every single one of those 440 ended up getting the virus. Now, even if the death rate was twice the state average, due to folks that experience homelessness having less access to healthcare and many underlying conditions that worsen COVID-19, we're in the 30-40 range. All of those lives are valuable and worth taking extra effort to save, but in no way would the virus "wipe out" people experiencing homelessness.
The community is now focused more on making sure that hundreds and thousands of folks who haven't made rent payments in April and May won't get evicted and become homeless when the eviction moratorium expires in June.