Why do astronauts float if there is still gravity in orbit?
Because they are falling — continuously — and so is everything around them. The common phrase "zero gravity" is simply wrong about the physics.
Gravity at the International Space Station is roughly 90% of its strength at the surface. At around 400km altitude, that is barely above the surface in gravitational terms, since the relevant distance is measured from the Earth's centre, not from the ground. Gravity does not switch off at some boundary.
What orbiting actually is. An orbit is a state of continuous freefall in which you keep missing the ground. Newton's cannonball is the classic explanation: fire a cannonball horizontally and it falls and lands. Fire it faster and it lands further away. Fire it fast enough — around 7.8 km/s at that altitude — and the surface curves away exactly as fast as the ball falls toward it. It never lands, and it never stops falling.
Why that produces weightlessness. Weight is not gravity — it is the force you feel from something supporting you. Standing on the ground, gravity pulls you down and the floor pushes back; you feel that contact force. In freefall nothing supports you, so there is no contact force and nothing to feel.
This is why a falling lift or the top of a rollercoaster produces the same brief sensation, and why aircraft flying parabolic arcs — the "vomit comet" — are used to train astronauts and film weightless scenes. Those aircraft are simply falling, on a carefully flown trajectory.
The correct term is microgravity, not zero gravity. There are tiny residual accelerations from atmospheric drag, tidal effects across the station's length, and crew movement — small but measurable, and relevant to sensitive experiments.
A useful test of understanding: if gravity vanished at the ISS, it would not orbit. It would travel in a straight line and leave.