They absolutely could have done Level 13.

At Level 14, there are numerous subclass features, that would have been a pain to implement, but for all the spellcasting classes, level 13 just has the Level 7 spells.

But, like they did with Level 6, they could have just offered some of the iconic spells at Level 7.

• Bard -Mordenkenian's Sword, Regenerate
• Cleric - Divine Word, Firestorm, Regenerate
• Druid -Firestorm, Regenerate
• Sorcerer -Finger of Death, Firestorm, Power Word: Pain, Prismatic Spray
• Warlock -Finger of Death, Power Word: Pain
• Wizard -Finger of Death, Mordenkenian's Sword , Power Word: Pain, Prismatic Spray

For the sake of 8 spells, which aren't game breaking, they could have put in Level 7 capacity.

Looking at martial characters:

• Barbarian - Brutal Critical (2 die) - an increase to a sub-feature, that they already get at level 9.
• Fighter - Indomitable x 2 - an increase to a sub-feature, that they already get at level 9.
• Monk - Tongue of the Sun and Moon - mainly a ribbon feature, but this could have deciphered the odd phrase and provided advantage when speaking to Drow, Infernal, Aberrations, Celestial by speaking in what they perceive as their own language etc.
• Paladin - Banishment and Death Ward spells are all in game. Staggering Smite seems an easy one to put in (as do the Aura spells tbh).
• Ranger - Dominate Beast, Freedom of Movement, Grasping Vine, Stoneskin are all in game.
• Rogue - is the most tricky one, as they get subclass features. For Arcane Trickster, versatile hand seems fine, Assassin could have Imposter to give Advantage on deception checks, Thief's use Magic Device would be awesome (mainly to get past racial restrictions for items), Swashbuckler(advantage on acrobatics and athletics). All seem very doable.

I think it would have just been nice to add these in. Again, not game breaking, but would give that extra feeling of being more powerful.