QUANTUM COMPUTERS
You are comparing two different orders of magnitude in processing power.
Integrated Circuits (IC) may suffice a drone for sensors, attitude control, communications, etc. Even for basic mission fulfillment tasks.
Their processing usually requires little power, is small, lightweight, cheap.
Then you have proper computers, quantum computers (QC), homes to AIs. Mean beasts that may solve NP problems so blazing fast they may burn the fingers on your keyboard.
They drive billions of devices connected (aka IOT), set missions both to drones and human crews, assess sensor reports, analyze and upgrade themselves constantly. They run the world.
They run on the fastest possible hardware for the task: quantum computers.
This technology just does not handle well miniaturization.
Note that current quantum processors are still a far cry from the marvels of your future.
In 2019, Sycamore completed a task in 200 seconds that Google claimed, in a Nature paper, would take a state-of-the-art supercomputer 10,000 years to finish.
Sycamore processor - wikipedia
But Sycamore has a measle, pitiful, laughable 53 qubits.
QC processing power is measured in Giga qubits, a whole other world.
They need a complex apparatus to work, a controlled environment, are very expensive and they are large.
Most of all they need cold.
You see traditional racks with 1000W processors can be quite a challenge too cool.
But in quantum chips power consumption is very, very low.
There is a catch though, quantum processors need to be kept at a very low, very steady temperature. Even small temperature (10 millikelvin!) increases can render the entire system unworkable.
To keep systems in a quantum state, designers have to minimize the risk of anything disrupting the fragile position. The slightest temperature increase can mean that atoms and molecules move around too much, potentially causing a quantum bit (qubit)'s voltage to spike, and flip from one quantum state to another.
See Cooling Quantum Computers
They also need to be shielded by external factors that may induce decoherence, the information in the quantum system can become randomized or totally erased. That is in general referred to as noise.
Noise refers to the multiple factors that can affect the accuracy of the calculations a quantum computer performs. Quantum computers are susceptible to noise from various sources, like disturbances in Earth’s magnetic field, local radiation from Wi-Fi or mobile phones, cosmic rays, and even the influence that neighboring qubits –the building blocks of a quantum computer– exert on each other by mere proximity. These disruptions cause the information an idle qubit holds to fade away.
Noise in quantum computing
Quantum error correction (QEC) deals with noise reduction but it is necessary to have the processors in a controlled environment.
In short the need for extremely low, strictly controlled temperatures and a safely controlled environment prevent miniaturization of QC.
But they are great, every mad scientist wet dream.
And are sold with all sorts of blinking leds for the retro afficionados.
For reference:
Quantum Computing