Why is uk leaving the european union?

There is 2 main reasons why:

Economics:
Opponents of the EU argued that there is a dysfunctional economic entity. The EU failed to address the economic problems that had been developing since 2008, for example, 20% unemployment in southern Europe.

Sovereignty:
The rise of nationalism across the world. There’s a growing distrust of multinational financial, trade, and defense organizations created after World War II. For example, the EU, the IMF, and NATO.
Many who oppose the EU believe these institutions no longer serve a purpose. Not only that, but also that these organizations take control away from individual nations. Mistrust and fear of losing control made Brexit a reasonable solution to the UK.