ROBLOX does not have a specific reason why tickets were removed, destroyed, vanished, unusable. I looked up on the internet, and ROBLOX Wikia already found out why.
According to a post, it said that multiple factors caused tickets to go out of existence.
Table of Contents
At the beginning
Tickets started out at 5-10 TIX per R$. People who sold clothing and other items can sell it at any price.
ROBLOX made an update called Price Floor, it kept clothing at a specific price, depending on what type of clothing.
It affected T-Shirts, Shirts, and Pants.
1 R$ was now 18 TIX.
Developers Exchange (DevEx)
OBCs can trade ROBUX for U.S. Currency.
Though, once a person traded their R$ for real $, the R$ vanished, therefore, rising the cost 23 TIX for 1 R$.
ROBUX started disappearing from ROBLOX.
ROBLOX then discontinued tickets in order to prevent this from happening. If they continued to support tickets later on, then ROBLOX would shut down permanently. In order to keep ROBLOX running in the future, they had to discontinue tickets. It is hard to explain why.
Tickets were discontinued for many reasons, including the one to keep ROBLOX running in the near future.
If you have questions, do not ask me. Email firstname.lastname@example.org. They can solve your questions about tickets, and other things about ROBLOX.
Saying Goodbye to Tickets: http://blog.roblox.com/2016/03/saying-goodbye-to-tickets/