Because you are setting a floor. First, if you make a minimum income of 1,000, anyone making less or around that will demand a raise, after all why would I work if I could get the same amount loafing around the house. So the mid-lower tiers get a raise, and now the tier above that doesn't want to make the same as the lower tier and they get a raise, so there is inflation as everyone suddenly as a lot more money to spend.

Which feels great for maybe a month or maybe a day, but with everyone making more money, goods in that area will rise in price, as landlords find they can make more money etc... and then we're back in the same situation. In theory goods outside of this economy with a floor would remain the same, but since the people that need to import it, store it, etc... would also be affected by the inflation in the end it doesn't net anything.

TLDR; a basic income is an illusion, and inflation would quickly catch up.