Why Pluto is not a part of solar system now?
It is part of the Solar System. The Solar System includes planets, moons, comets, asteroids, a star and dwarf planets like Pluto. It's just not a planet. It never was a planet (according to the new definition).
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
Pluto is no longer considered a planet because it does not fulfill the criteria, but Pluto never stopped being considered a part of the solar system. It still orbits the same star, although it is a very distant orbit.
The old name of our star, which we sometimes call the "Sun" is "Sol". Sol has a number of planets, planetesimals, and rocks orbiting it. This is therefore called the "Sol"-ar system, or simply, the Solar System. Pluto is now classified as a "dwarf planet". Pluto may be a part of the Kuyper Belt at the far edge of our Solar System.