Because it is a very well known software company. I have never used Citrix, but to me its as a name the same way as VMWare, SAP, EMC, Salesforce and Intuit is.
If you have a datacenter, it has a good shot of running some VMware.
If you run logistics at almost any medium sized or bigger company, you’re using SAP or Oracle.
If you have a group of employees that sell things, Salesforce is common.
This is the software that literally runs the world. If you’re not familiar with it, then you’re ignorant about how the world works. Nothing wrong with that of course, but I don’t understand the pride in it. Especially on HN, where Hackers tend to be curious.
Either it's completely automatic (like most PAYE employees in the UK), or the tax system is simplified to point where it's a simple form to complete.