[mcraiha]

If you mean liquid metal vs. paste, then the answer is money. Liquid metal is more expensive and harder to apply.

If you mean solder vs. paste then "Micro cracks in solder preforms can damage the CPU permanently after a certain amount of thermal cycles and time. Conventional thermal paste doesn’t perform as good as the solder preform but it should have a longer durability – especially for small size DIE CPUs." http://overclocking.guide/the-truth-about-cpu-soldering/