Nobody but Game Freak knows this. Either Game Freak wanted it this way, or there wasn't enough free data space on the cartridge to input data for 5 new trainers (E4 + champion).
However, if you want a challenge, you can go the Battle Chateau. Once the rank of Duke/Duchess is achieved, the Elite Four will begin to go to the Chateau, but with only three Pokemon. A Red or Black Writ of Challenge will increase the levels of their Pokemon to above what they were before. If you achieve the rank of Grand Duke/Duchess, Diantha will also appear and you can use the same writs to increase her Pokemon's levels to above what they were before.
Also, the Elite Four in Kalos (for the most part) have higher leveled Pokemon in their Elite Four than their Kanto, Johto, Hoenn, Sinnoh and Unova counterparts due to how late you face them in the game. Let's take a look at their Champions and their highest leveled Pokemon on their first runs:
Blue (R/B/G)
Level 65
Blue (Yellow) 
Level 65
Lance (G/S/C)
Level 50
Steven (R/S) 
Level 58
Wallace (Emerald) 
Level 58
Blue (FR/LG) 
Level 65
Cynthia (D/P) 
Level 66
Cynthia (Platinum) Level 62
Alder (B/W) 
Level 77
Iris (B2/W2) 
Level 59
Diantha (X/Y) 
Level 68
So, besides Alder, Diantha has the highest leveled 'signature' Pokemon than any other champion. (Alder has higher leveled Pokemon due to the fact N interrupts your initial battle).
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