There is no one single reason.
NH4, vegetative vs sexual reproduction, CO2, light differences etc all play roles with specific species of algae.
NH4 induces algae whereas NO3 does not over a an extremely wide range.
Too little PO4, NO3, Fe etc can induce algae as well, as can variations in CO2 ppm under higher light.
So too little nutrients can induce BBA, GSA, GDA, BGA, hair algae.
Not so much excess.
Algae have been around a long time, they have little reason to worry about plants outcompeteing them, they will wait till the plants die back and then they will take over in natural systems and many natural system don't have plants anyway, there's not enough nutrients to support them.
Plants namely grow fast when they have lots of light/CO2 and nutrients.
Just like in our tanks.
More plants=> less algae.
Algae have spores waiting and hanging out till things get good and the environment is ripe for them to grow well.
Plant seeds and algae spores just don't sprout and germinate for no good reason.
These are adaptations to get through tough times ahead.
Most desert plants are annuals and bloom only after a rain.
Algae, same type of thing, you add NH4, which is a good indicator of good growing conditions, and the algae will take off.
Some algae like lower nutrient levels before spores germinate, some like it when CO2 is all over, some when PO4 is low etc.
I'll go into detail later about all that, it's a 2-3 part series on algae I'll write up.