Why does Earth still have an active, Hot interior core, but Mars and Venus have none?
Why does Earth still have an active, hot interior core, but Mars and Venus have none?
This is incorrect. Mars and Venus almost certainly have hot molten outer cores, just as does the Earth. The evidence for this lies in the time variations of the planets' gravitational fields. The gravitational field of a perfectly rigid body is constant. Planets aren't perfectly rigid. They instead are subject to tides. These solid body tides result in small time variations in a planet's gravitational field.
The variations in a planet's gravitational field provide a means for inferring characteristics of the planet. In particular, both Venus' and Mars' $k_2$ tidal Love number are consistent with a planet with a partially molten core but inconsistent with a planet with a solid core. The cores of Venus and Mars most likely look a lot like ours: a molten outer core, possibly surrounding a solid inner core. We'll need to place seismometers all over the surface of those planets to determine if they have a solid inner core.