Why don't you post the Unearthed Arcana Options in the Homebrew Section?

Seems like a no-brainer to me.  Why don't you take the extra step to post the Unearthed Arcana optional Subclasses into the Homebrew area so we can more easily test them out?

Is there a solid reason why this hasn't been done?

It was done for years under the old management. Then they (still the old management) found that many of the the things they put up would not get through the playtest to be published, or would be completely changed so they’d effectively have to be posted again. They decided it wasn’t worth the expense of it. Since WotC bought the site, thus far they’ve agreed. 

This seems silly to post the UA in PDF form only on a site that's function is to put Dungeons and Dragons at your digital fingertips and to integrate it seamlessly. 
What is the expense?  An hour's worth of labor?  Two? (Imagine these last lines spoken in Lucille Bluth's voice.)

Seriously though, am I wrong that doing so would result in far more people trying it out?

To assume that integrating each UA game option into the system for use in the character builder would be "an hours worth of labour? Two?" is hilariously naive. We're not talking something a user has chucked into the homebrew tools without a care or consideration. 

D&D Beyond stopped integrating UA into the character builder because in their own words it wasn't a star e investment of time or effort.