One of the difference between Roman (aka civil) law and English (aka common) law is the that precent matters differently.
Lots of countries in Europe are based on civil aka Roman law.
The Act of Union in 1707 Scottish law was guaranteed to continue after the union, that was a demand. Why didn't they change it? Probably for the same reason the USA adopt Mexican law? Why should they? It's a different country with different legal systems.
Bad analogy. The US and Mexico are not connected by an Act of Union.
And actually - parts of Mexican law were incorporated by states in the Southwest, where they made sense: primarily land-use and water rights.
It's a different country with different legal systems.
Act of Union - as I read it - united Scotland and England into the same Kingdom. You guys shared a flag, your armies are the same, the currency is the same.
A difference with no distinction.
Now, a prickly Scot could want to keep his oddball legal system and more power to him. But don't get all irate if the rest of the world looks at that choice and wonders that two countries that share currency, an army, a monarch, and etc should not also share the same legal system.
These rules about what makes a country are mostly arbitrary and are often made for political reasons to either give power or take power from certain people.
Says on Wikipedia[0] that it's a hybrid of Roman and English law. Why did they even use Roman law at all? The Roman Empire had been gone for several centuries, and England was in control after the 1707 Acts of Union, so it's surprising they didn't reform the system to follow English procedures.
0: http://en.wikipedia.org/wiki/Scottish_law