It's easier to make tiny pixels with OLED because organic materials can be printed directly onto surfaces, while MicroLED requires physically placing millions of tiny inorganic LEDs one by one, which is far more complex and expensive.
Assuming you meant to ask why organic LED screens were widely available before the inorganic micro-LED screens.
Even a small screen is made of a very large number of pixels. Each of these pixels contains an electronic circuit which turns it on and off. So this pixel array control circuit must be fabricated first, using conventional thin film technology. Then one has to add the LEDs.
With the organic LEDs, the LEDs can be built in additional layers on top of the electronics of the pixel array, without destroying it.
Ordinary inorganic LEDs are made from materials that require a different, incompatible process, and they must be fabricated separately, and then transferred to each pixel. This is very hard to do quickly, inexpensively and with acceptable quality.
You can basically print OLEDs onto a plastic sheet with what is essentially an ink-jet printer and then glue that sheet to a piece of glass to make a display.
The process of manufacturing conventional LEDs meanwhile involves things like photolithography, vacuum ovens etc. making the process overall much more complicated and prone to failure.