I cannot speak to the design choice to not include XIRR as a function in Tableau Desktop. The Tableau website has an Ideas section where customers can add, comment, and vote on features they would like to see in the product. Votes are used to help prioritize new features. Our Development team also loves to see everyone's use cases in the comments, which helps them better design features that will meet the most possible needs.
I recommend voting/commenting on https://community.tableau.com/ideas/2893
XIRR is an iterative function, which it is not possible to calculate with standard Tableau calculations. Check out this blog post that discusses how to calculate XIRR using R How to Calculate Internal Rate of Return (IRR) using R in Tableau
Hope this helps
I just posted a solution on: https://community.tableau.com/ideas/2893#comment-35913
After trying R, Python, Custom Calculations... I just decided to try with a Tableau Extension and that did the trick!
Thanks for sharing this, David