1- Low quality or damaged wire
The most common reason for this is your cable, a charging cable that has higher resistance (worse thinner or damaged wire) will tell the phone to charge slower, reason for that is that your phone does not want to overwhelm a cable beyond it’s capacity and cause it to burn or even cause fires, but how does the phone know, well, ohm’s law will let it know, a voltage drop at the wire end once a load is attached (the charging is a load) tells it all it needs to know. then the phone can determine a safe current to draw.
2- Phone is already nearing a full charge
Because of how lithium batteries work, a phone can not charge the upper 10% (Or more) at full speed, speed is reduced as the phone gets full to protect the battery from degradation
3- Battery is overheating
Your phone will charge a hot battery slower, so if your phone is sitting in sunlight or is hot in general, it will charge at reduced speeds, this as well is to protect the battery from degradation or even becoming a hazard.
4- Your power brick capabilities
Power bricks are voltage and current regulated power supplies, but they lose capacity over time and degrade, sometimes, their connection with the cable is not clean, or the plating on the pins has worn out, or the spring like mechanism of the pins no longer holds the pin tight against the pins on the USB cable, sometimes it has low capacity to begin with by design
5- Your phone has a Quick Charge capability but your wall adapter does not
Many modern phones, especially on the higher ends have a fast charging feature, where the power brick provides a higher voltage rather than a higher current, your phone will first negotiate this capability with the charger, if your charger does not support this feature, rest assured, it won’t work, and the phone will fall back to 5V charging.
If you have any questions about this matter, feel free to ask me in the comments section