...you just play Frost?
Like, Frost Mage tree has tons of beneficial dps talents, it's not like "Fire = damage ; Frost = defense".
If you already chosen talents that improves your frostbolt, but then to also decide to improve your Fireball, you will not see an actual damage gain until your Fireball surpasses Frostbolt in terms of damage.
Once that happens, all Frostbolt has left is the slow, where an already decent version could just be applied by the baseline version.
You're just biting your own tail, if you improve Frostbolt, spamming Frostbolt is the most effective way to deal damage, if you improve Fireball, spamming Fireball is the most effective way to deal damage.
If you improve both, you are not reaping the full benefit of your talentpoints and are weaker than someone who, for example, went deep Frost.
Because their Frostbolt is much stronger than yours and Fireball requires deep fire talentpoints in order to be competitive with a Frostbolt that has all the improvements from Frost.
Improving both abilities at the same time is anti-synergistic, therefore bound to end up being inferior.