why a transformer feeder is not provided with line isolator

Query is not crystal clear to understand?

May be you can explain with sketch/ SLD

The purpose of an isolator is to enable the equipment or line to be taken out of service for maintenance in a safe manner. Perhaps in your system this can be done without the isolator in that particular location, hence the design engineer left it out.

It is good practice to have isolators in an outdoor HV switchyard because they provide a 'visible break' in the circuit and thus make it easy to visually ascertain that the equipment (circuit breaker, line, etc) is safe to work on.

You have to give more information (single line diagram?) before we can say anything more specific.