I'm playing what's effectively a counter-caster, and a spell I noticed that is on both the cleric and druid spell lists is the cantrip Resistance. This spell gives a bonus to a single saving throw, and would fit well with what I'm playing. However, it's not on the mage spell list. Why? The only reason I could think of is the roles of clerics and druids versus the role of a mage, but that is fairly minor especially given that it's a cantrip.
Only the designers can accurately answer this; our answers would only be speculations.
It seems to me that each class's playtest spell list is made more distinct than in 3.x... by cutting spells from the list.
Bards no longer have Grease, Haste, Mirror Image, Blink, or other "mage-like" spells. Less support and buff, which renders them a lot more trick-oriented.
Clerics lost Divine Favor, Magic Weapon, Wall of Stone, Astral Projection, etc. Less fighting and controlling, and slightly more about protection and anti-undead/outsider.
Even Paladins and Rangers now see a very different and unique spell list (aura, smite, arrow spells).
Mages lost Detect Poison, Tongues (now Holy Accord), Daylight, and your example of Resistance. They are now slightly more focused on control (especially since they can only concentrate on one buff).
With the increased availability of healing/restorative spells (reducing diversity), and the introduction of sub-class spell list (further reducing diversity), I would say this is designed intentionally to keep the base classes more distinguished.
In fact, since cantrips can be cast at-will, they are the most used spells. Do you light a camp fire with Thaumaturgy, Fire Seeds, or Prestidigitation? Do you pray for divine Guidance before you do anything? Perhaps you Ray of Frost all your drinks. In real play, they really say a lot about your class.
Or you can work out an abjuration mage with your DM, with the Resistance cantrip as a class feature.