Why?

Most of the time "developers" don't make the time to learn CSS. Some people consider CSS "messy", however a better description is: "quirky"...
There are many CSS/UI frameworks that attempt to give you an out-of-the box "solution" for CSS, and a few have been successful. These are a few we have used (but there are many more to chose from):

• Bootstrap: http://getbootstrap.com/ - by far the most popular, but it has morphed into a mega-framework with lots of CSS & JS ...
• Flat-UI: http://designmodo.github.io/Flat-UI/ - we really like Flat-UI and have used it a few times, especially like the form elements. Sadly, the "wow" comes at a price. In addition to including bootsrap.min.css (22kb) your users still need to download flat-ui.min.css (27kb) and flat-ui.min.js (73kb) ... all-in you're going to force people accessing your site/app on a mobile connection to wait 2 seconds before the page is loaded...
• Pure CSS: http://purecss.io - Yahoo's answer to Bootstrap. Much smaller (4.0KB* minified and gzipped) and CSS-only.
• Foundation: http://foundation.zurb.com/ - probably the most feature-rich UI framework. like bootstrap, has both CSS and JS.
• UI Kit: http://getuikit.com/ - lots of "fancy" (JS enhancements) some people will find "reasons" to use. which comes at a price.
• Material Design (Light): https://getmdl.io/components/index.html used it in a couple of client projects. hideous naming convention and difficult to override the JS interactions.
• Skeleton: http://getskeleton.com/ - v. lightweight. though appears to be un-maintained at time of writing.

Why Tachyons?

Tachyons gives you less. Much less than virtually all the other frameworks. Why is this a good thing...?

Read the Principals: http://tachyons.io/#principles and see if you agree ...