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.
So rather than 0%-20%-40% income tax bands, we are talking about 20%-40%-60% tax - which looks ridiculously high on the face of it, but has no effect on after-tax income of the average earner after the UBI.
Because of the shocking sticker effect of the taxes, my view is that it is a lot better to position UBI as "negative income tax for low earners" if you want to get it adopted...
I don't see how that's true. Isn't the UBI supposed to replace ALL of those programmes with a single number for everyone that is sufficient for everyone? If the UBI provides enough for food and shelter you shouldn't need housing benefits. Disability may be different I suppose but if you cannot meet your needs with UBI due to a disability I think that's something you could probably roll into the NHS where equipment you need but cannot afford is provided to you by them (I haven't really thought that through though it's a very good point you've brought up).
Otherwise you need a flat subsidy much higher than £550 per month to properly replace the existing subsidy in all parts of the country, which would cost an awful lot more in taxes and make it a very comfortable living in other parts of the country.
Of course, there's an argument that subsidising people that clearly couldn't afford to live in the area they grew up in isn't a particularly good use of funds (compared with subsidising gap years, early retirements and stay at home partners?) but I can't see the left buying it. Or anyone on the right being keen to find the extra gross £140bn pa you still need to find even once you've reduced disability and housing spend to zero.
I think there are lots of ways to adjust current spending and make it work. My personal idea would not fly with the American public though (reduce defence spending by 80% and raise taxes - especially for higher earners).
I can't see how they could believe this. First of all a minimum wage becomes unimportant as you should hypothetically have enough money to support the kind of lifestyle a minimum wage job would provide without working. Secondly, as it's no longer financially critical to them, people won't be as inclined to take on minimum wage type jobs - which will force the wage up anyway so that the business can attract employees. So it should take care of itself. As for paid family leave (I presume they mean maternity/paternity leave?) you won't need that as your basic income will ensure you still have money coming in. And if the company wants to retain your services after your leave they will offer it anyway. The key point in these examples is that even if you don't get a higher minimum wage or paid family leave it's no longer going to have a big effect on you as you have your UBI to rely on.
Also, it doesn't seem to me like there is a left/right split on this elsewhere in the world. This leads me to believe that the problem is the highly partisan US political system. The right is obviously going to support UBI as it would significantly reduce government size - the left can't be seen to be agreeing with the right. I think it's a kind of childishly schoolyard thing you see a lot in US politics (he likes that so I don't).