Backend server side work requires a deeper knowledge of things like Threads, sockets, processes etc. Some knowledge of languages like C etc is generally a requirement.
In general front end work requires knowledge of HTML, CSS etc. which kids do in high school.
Having said that they are both separate skill sets and supply and demand probably plays a part. There are a lot of people who want to be front end people - designers etc, you get to wear cool designer glasses and have meetings about user experience, while sipping lattes.
Back end guys sit with workstations and talk about boring things like sockets and memory usage, so there's less people who want to do this stuff.
Thats probably why full stack developers are more expensive and rarer you need a bit of each.