I saw a discussion related to this some time back.
In summary, those energy types were added after the most common monsters were written, so resistance to those energies is rare. Thus, they are "unbalanced." That's why they weren't included on the magus list in the first place.
I personally don't like arbitrary balance decisions, but that's the thinking.
With that said, if anyone knows of options, I'm interested too.
I'm not sure radiant or necrotic are magus options in Paizo publications, but adding acid damage to a weapon is as simple as wearing deliquescent gloves. However, there are some other ways to expand on the magus abilities.
Devoted Blade (Su) (Ultimate Combat pg. 54): Whenever the magus enhances his weapon using his arcane pool, he may spend 1 additional point from his arcane pool to add either the anarchic, axiomatic, holy, or unholy special ability to the list of available options. A magus may only add one of these abilities if it matches his own alignment. The magus must be at least 12th level before selecting this arcana.
Ghost Blade (Su) (Ultimate Combat pg. 54): Whenever the magus enchants his weapon using his arcane pool, he may spend 1 additional point from his arcane pool to add the brilliant energy and ghost touch special abilities to the list of available options. The magus must be at least 9th level before selecting this arcana.
Pool Ray (Ranged Tactics Toolbox pg. 13): The magus can expend 1 point from his arcane pool as a standard action to infuse a ranged weapon with elemental power. As a free action while making an attack roll with the infused ranged weapon, after the attack roll has been made but before the results are revealed by the GM, the magus can release the charge and cause his attack to deal 1d6 points of energy damage (acid, cold, electricity, or fire, chosen when he spends the arcane pool point to activate this ability). If he misses with this attack, the charge is lost and the effect is wasted. If the magus has the ranged spellstrike class feature (such as through the myrmidarch archetype), he can use this ability with ranged spellstrike to treat it as its own ranged touch attack. At 6th level and every 3 levels thereafter, the amount of damage dealt by this attack increases by 1d6.
There is also the elemental knight archetype (must be suli).