It's considered a distraction because it's a political pipe dream that has a very low chance of being implemented.
The reason the left doesn't like it is fourfold:
* The pretext for basic income presumes that we have a lack of spending because we've run out of things to spend money on. This couldn't be further from the truth. American infrastructure is crumbling and yet politicians still want to restrict necessary spending, choking off demand and killing jobs.
* The pretext for basic income also presumes that we have a wide variety of cheap goods that don't need much US labor because of efficient markets and automation. We don't. We have a wide variety of cheap goods not requiring US labor because China intentionally undervalues its currency. China can pull the plug on that at any point and trigger a torrential level of inflation in the US.
* The last time the US faced this problem of high unemployment due to artificially suppressed demand (in the 30s) it was successfully fixed with federal job guarantees that ended up providing useful employment and getting a huge amount of useful infrastructure built that we still use today.
* However much you try to deny it or dress it up, people actually want jobs - for more than just the income.
