Why differentiated cells can't "undifferentiate" (under normal conditions)?
This "undifferentiation" is actually possible and it is known as induced pluripotent stem cell (iPSC). The first "undifferentiation" cells i.e. iPSCs were mouse fibroblasts created in 2006 by researchers in Shina Yamanaka's lab who got the Nobel prize in 2012 for this work.

You can find further reading on this topic on the according Wikipedia-Page or more scientific details on the according nature.com topic.