Why is Pluto now classified as a dwarf planet?
Objects bigger than Pluto have been discovered.
There are several reasons for this.
Firstly Astronomers have found objects bigger than Pluto in the outer solar system, perhaps 3 to 5 objects are bigger than Pluto.
For an object to be considered as a planet, it must be big enough to be round by it's own gravity. Pluto satisfies this condition.
For an Object to be considered a planet, it must have cleared it's zone of solar system debris early in the formation of the solar system, but Pluto has no debris indicating that it was not formed at the same time as other planets but was captured in the later years of the Solar system.