Because the 'skin' of the base components is not the problem most of these frameworks are trying to solve.
This framework seems to have very little to do with the look and feel you are after. Hence it's light filesize, and it's advocacy for a less opinionated style.
"...provides layout and styling for native HTML elements, plus the most common UI components. It's what you need, without the cruft."
"...minimal styles and encourages you to write your application styles on top of it. It's designed to get out of your way and makes it easy to override styles."
Bootstrap on the other hand, has a fairly opinionated design. That is to say, it does a lot of the 'look and feel' for you, and overriding those styles to create your own skin takes a fair amount of work. Pure seems to aim at making a uniform base of elements, without designing a lot of the skin... encouraging you to extend base classes more than trying to override them. And I applaud these types of frameworks.
Lastly, IMHO the solution for grid systems is different in many of these frameworks, and a lot of it comes down to personal preference and your project's requirements. This is a pretty big differentiation point for some people. Many frameworks have opinionated grid systems -- box-sizing: border-box, fluid vs. adaptive, first/last class names required on columns within a row, etc.
This framework seems to have very little to do with the look and feel you are after. Hence it's light filesize, and it's advocacy for a less opinionated style.
"...provides layout and styling for native HTML elements, plus the most common UI components. It's what you need, without the cruft."
"...minimal styles and encourages you to write your application styles on top of it. It's designed to get out of your way and makes it easy to override styles."
Bootstrap on the other hand, has a fairly opinionated design. That is to say, it does a lot of the 'look and feel' for you, and overriding those styles to create your own skin takes a fair amount of work. Pure seems to aim at making a uniform base of elements, without designing a lot of the skin... encouraging you to extend base classes more than trying to override them. And I applaud these types of frameworks.
Lastly, IMHO the solution for grid systems is different in many of these frameworks, and a lot of it comes down to personal preference and your project's requirements. This is a pretty big differentiation point for some people. Many frameworks have opinionated grid systems -- box-sizing: border-box, fluid vs. adaptive, first/last class names required on columns within a row, etc.