The Persian Empires have fairly regularly held land in Southern Central Asia and Afghanistan and so in the north of Afghanistan and Tajikistan the majority speak Persian as their first language. However, as Afghanistan is not a fully Persian nation it gives the impression that Tajikistan is separated from other Persian speakers despite them being connected through Afghanistan.
Kazakstan is my favorite stan. It's a beautiful country with a beautiful culture. It has all kinds of fun stuff about it, like having one of the largest inland lakes, the largest landlocked country, and being the last country to leave the USSR. Fun!