Problem in, there's no real reason to call them Kender, it doesn't really bring anything to the table, whereas changing the name from Halfling to anything else will just annoy all of the Grognards, who'll complain about them "forcing flavor" or "change for the sake of change" (and they'd have a point with the second one), not to mention that they're called halflings in FR, Eberron, Darksun etc which means the settings would have to change them back.
Also, Halfling is known outside of D&D, Kender isn't, so it makes it that bit harder for new players to get in.
But mostly, saying that "4E Halflings are Kender with a fondness for rivers and other waterways" isn't really the case, it should be pointed out that they said that 3E Halflings are already based off of Kender, but with the annoying things removed. If you make them more like Kender (an call them Kender) you get the annoying players who use it as an excuse to steal everybody's stuff and and get the entire party killed through utter disrespect for danger because "that's what Kenders do".
And no-one wants to go back to that.