I believe the reason is that the wallet itself, we all know every ERC-20 address is supported on different chains. Then, is really makes them easy to set up just a single wallet but support receive from the different chains they're accepted.
I agree that adding new chains and new tokens would be a lot of work that is not really needed for the time being. Lets realize that this place gets 20k+ people all at once online at sometimes, which means that they already have a lot of people they have to care for and they are getting ready for even more when the time comes.
It means that if they started working on something else, it would both bottleneck all the improvement they have on the works right now, but it would also cause a lot of people to back out as well. I personally believe that adding new tokens and chains is futile work when it is not that big and we all know TRX is not that big at all, look at cmc if you want to make sure.