Obviously, it would be crazy to have infinite DM but why is it on a meter? Like what does she use when putting up DM that runs out? Can’t she just shoot down projectiles for as long as she wants?
If we’re talking fluff-wise, then simple physical stamina/exertion. Human body can only go so fast for so long.
She used to press buttons but that animation went to missiles
In a word, balance.
Though from a “lore” perspective, it can just be explained that the hardware needed to project the matrix overheats if it’s used too much and has to be allowed to cool down
Her APM magic mana runs out.
It’s mountain dew. She needs it to keep her APM that high.
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Lol she has an IV line of Mountain Dew concentrate running into her body lol.
for the same reason genji can only use his blade 6 seconds at a time on a very sporadic timer and rein can’t just go around earthshattering all the time - balance reasons
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Are you trying to find some logic in no-brain fanservice hero?
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Ya lore wise D.va is in full control of Defense Matrix, that why she says, “Lets increase my APM”, she is shooting down all the projectiles not a computer
Back when reaper used to eat souls jeff said that not all things that occur ingame necessarily make logical sense or are canon, they’re just there for the sake of balance
Defence Matrix is basically D.Va shooting down incoming projectiles with turrets. Well, it used to be turrets but they’re now used for missiles. In any case she needs ammo to use it and the bar could represent the ammo count I suppose.
Her ADD medicine wears off. 
She can’t keep on that ridiculous level of concentration any longer?
She uses lasers and her mech has a reactor in it. Reactors can give consistent energy for decades, so that can’t be it.