Politics, I expect that service to have relatively low ridership (why not get onto a plane for the same price and arrive 2h earlier at probably more convenient times)
It also builds ridership to justify more investment in infrastructure though. It's only 1h44 on LGV Est to Strasbourg, then the lines just across the German border towards Frankfurt are being upgraded towards a max speed of 155mph up to Frankfurt, then the 186mph Gelnhausen to Fulda HSL is being planned and there's an approx. 60 mile gap on the classic network between Fulda and near Eisenach, where upgrading isn't planned as far as I can see, before you reach the upgraded and new HSL route all the way to Berlin.
If you managed to close that gap and ended up with an overall time of 4hr30-5hr after a decade of upgrades and new lines, then that is an attractive proposition. It's going to be at least 3hr15mins from central Paris to central Berlin once you've accounted for the RER to CDG or metro to Orly (according to Google around 30 mins Chatelet Les Halles to CDG and 27 Chatelet to Orly), security and check in time (40 mins for Air France/Transavia or at the gate 30 mins before for easyJet), the flight and then the transport in from Brandenburg airport to central Berlin (min 25-30 mins, according to Google).