TCR is a car specification, not just a single series. Having multiple series running the specification adds additional production for manufacturers to amortize car costs and the potential for profits incentivizes manufacturer participation.
It makes no sense to restrict TCR spec cars to one series. The TCR International Series is the top series and races across the Eurasian continent, but not every team interested in TCR can afford to traverse the world's largest continent or has the ability to compete at that level, hence we have (the wildly popular) TCR ADAC in Germany, as well as TCR STCC, TCR Benelux, TCR Asia, TCR Italy, TCR Middle East etc. TCR cars are also eligible to race in lower classes in VLN, Super Taikyu, 24H series, 24H TCE series and several stand-alone endurance races. Like GT3 and GT4, you can run a TCR across a number of series, formats and in a number of places, which the touring car racing world is all the better for.