Because the poll was flawed and tainted from day 1.Because they have been misled.
The company funding the poll is a well known far-right page whose owner has publicly railed against "woke culture", and the methodology involved primarily the use of landline phones, which as a marketer, I can safely tell you, residential users of landline phones tend to lean older and more conservative.
So even dems who answered would be conservative dems.
Lastly, anyone who thinks the democrats are a left wing party is sorely mistaken. Since 1992, it's been a centrist party with a few left leaning elements, but as far as where it goes on the L/R spectrum, is closer to the centrist Reagan-era republicans.
There is no party in the US for the left, which is why I'm a registered independent.