Why transformer rating in kva not kw?

Any Electrical Source shall mostly be rated in KVA. Transformer is also a Type of Electrical Power Source.

Further, any Electrical Load shall mostly be rated in HP or KW..

If Power Factor in the system remains near unity, Source KVA can be taken as KW also.

As Power Factors lowers in an Electrical System, Reduced KW of Source can be calculated as Source KVA  X  Power Factor.

Hence, an Electrical Technician / Engineer always keeps an eye on Power Factor of Its Electrical System.