Earlier quoted context omitted.
Citrix originally licensed the technology for the Remote Desktop Protocol (RDP) to Microsoft. RDP was a pared back version of Citrix, and the license terms left Citrix free to sell a with a more fully-featured virtualised desktop product running on top of Windows Server.
Wikipedia says "Remote Desktop Protocol (RDP) is a proprietary protocol developed by Microsoft" and that "[the] Terminal Services Edition of NT 4.0 relied on Citrix's MultiWin technology, previously provided as a part of Citrix WinFrame atop Windows NT 3.51, in order to support multiple users and login sessions simultaneously". Based on this it sounds to me like the protocol itself was the work of Microsoft and that…
One of the reasons companies licensed Citrix is because Citrix kept investing in ICA and it was about 2-3 times as bandwidth efficient. In the era of 64 Kbps ISDN WAN links this was critical.
Microsoft did something around 2008 and now RDP seems better overall. It can handle 4K at 30 fps without difficulty.
Meanwhile Citrix did weird random things to ICA that made it markedly worse.
It says a lot that all of the Citrix engineers I’ve worked with (including myself) prefer to RDP into a Citrix server instead of using its native ICA protocol!
Other deliberate breakage was Citrix deprecating SSL support directly on the session hosts. They did this to force customers to buy their overpriced Netscaler / ADC appliances. These slow down connections and can’t handle many 4K streams.
Generally they seem to have become an acquisition-driven company instead of an engineering-led one.