Why do Minoan cities had no fortifications?

Answer 1

to protect them from invasionsThe Romans

Answer 2

Not quite accurate, towards the end of their reign they did fortify some of their settlements, and their final redoubt/town was built in a very inhospitable, inaccessible location (Karfi) on the edge of the Lasithi plain.

But for the majority of their reign and most of their cities were indeed unfortified. This is due to their Navy and the fact that they were located in several islands too far for most people to navigate with the primitive craft available to them at the time. They had a very advanced maritime fleet that rivaled any other at the time and through their trading they would quickly determine if anyone would be able to also navigate the waters and act accordingly.

It wasn't until they were dealt several crippling blows, the biggest being the eruption on the Island of Thira/Santorini. Prior to the crucial eruption the island was the second most important of the Minoan settlements and was known as Calliste (the beautiful). This is widely thought to the origin of the Atlantis myth. The resulting eruption caused a terrible tsunami that damaged most of the Minoan cities on the northern coast of Crete (the center of the Minoan culture) as well as many of the islands and Coastlines in that part of the Aegean. This event crippled them sufficiently that they were vulnerable to a Mycenean invasion later on.

Also, this was a widely turbulent time throughout the Eastern Mediterrean Sea where many cultures were overwhelmed, and even Egypt, the biggest and most powerful nation at that time was hard pressed. The Thiran eruption is thought to have caused some of the biblical plagues and also may have started the wave of attacks from a group of peoples known as the Sea peoples. Remnants of the Minoans are possibly thought to be part of that and after being defeated by Egypt were settled on the coast of Judea and are thought to be the origin of the Philistines.