Why doesn't Venus have a magnetic field ?

In part because of its slow rotation (243 days) and its predicted lack of internal thermal convection, any liquid metallic portion of its core could not be rotating fast enough to generate a measurable global magnetic field.

Limits on magnetic field strength from Magellan magnetometer data are 0.000015 times Earth's field. Current theories of the formation and evolution of the terrestrial planets do support an Earth scale magnetic dipole (magnetic field) on Venus for perhaps the first billion years or so after formation. During that time, remnant thermal energy from the heat of creation probably drove the Venusian dynamo. After this heat was fully dissipated, there appear to have been no other internal processes with which to generate the convective motion needed to support a global field. Today, the only magnetic field Venus may have comes from the interaction between Venus's upper atmosphere and the solar wind. The interaction causes electric currents to flow in the upper atmosphere which then create a weak magnetic field that streams behind the planet like a comet's tail.