Because the 3 Degrees of Glory vary from each other - from the Most glorious (Celestial Kingdom) to the least (Telestial).  
 
Just like it says in 1 Cor 15:42 "There is one glory of the sun, and another glory of the moon, and another glory of the stars: for one star differeth from another star in glory." 
 
The Telestial Kingdom is a kingdom of Glory. It will be magnificent there. But the Terrestrial Kingdom is much more glorious than the Telestial. And in like manner the Celestial kingdom is much better / more glorious than the Terrestrial. 
 
Also in the celestial kingdom is the place where marriages are eternal and families are eternal.