Over the year due to revolution of Earth there are changes in the environment causes seasons to occur.
In a year there are four major seasons that are observed word wide ,that is, summer, winter, autumn and spring.
In different seasons different changes are observed in the environment such as changes in the temperature, plant germination and blooming.
Reason for fruits and flowers to be seasonal:
There are many varieties of plants that have different needs for germination, fruiting and flowering like temperature, moisture and oxygen.
Some plants need high temperature for fruiting and flowering such as watermelon and cucumber. Whereas carrots grow in cold temperature due to which it is mostly available in winter.
This is why most of the fruits and flowers are seasonal.
There are some plants which keeps on fruiting and flowering whole year.