Thermal paste is difficult to apply at large scale. It's cheaper and easier to use the thermal pads, even though they kind of suck. This is also why the stock cooler that came with your CPU came with one of those stupid pads.

The alternative is to use the paste and end up with the shitshow that was the nVidia 8000 series GPU debacle. People pulling them apart were uniformly aghast at the quality of the paste job, usually with big blobs of the stuff smeared all around the chips.

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/

Intel hasn't spoken about why they use the thermal interface material (TIM) they continue to use. Best guess anyone has is that there's a higher cost somehow with a soldered lid. I'm not completely sold on that, but Intel hasn't defended their practice in any way so it's open to interpretation.

I'd say cost. All the author did was remove the cpu lid and replace the cheaper thermal paste with a better after market brand.