What is the weakness of using Hi-Com TRIAC to drive motor compressor in refrigerator?
Triacs produce a lot of heat when on (which has to be dissipated somehow to keep the semiconductor from failing) and are generally less reliable than relays, especially when the power line contains transients. 

Relays have a definite life and will eventually wear out, however within their lifetimes they are very rugged and reliable. The switching rate for A/C equipment is low so the inexpensive and reliable relay is the sensible choice of manufacturers. Manufacturers can live with a definite life (though it means that service has to be accommodated) but they really, really hate infantile failures, especially those that occur during the warranty period. 

Even a minuscule percentage of field failures is extremely expensive, both in service costs and in damage to reputation. Even more so today in the age of online reviews and class-action suits.