It was hugely popular before being free. The problem was that some basic options for the classes that were strictly better or required for gameplay (the worst offender here is likely the Jackal[1]) were gated behind days of grinding or microtransactions. Newly introduced options usually had some severe balancing issues. The core audience and biggest advocates for the game were people that played Tribes and Tribes 2 decades earlier - they didn't really like that they couldn't buy the game outright and have all the content in it unlocked at a reasonable pace.

[1]: https://tribes.fandom.com/wiki/Jackal

IMO for two reasons, separated by player type. Casual players were turned off by the grind, and experienced players (read: players of former games) were turned off by the dumbed-down mechanics as compared to previous games.

By the latter I mean mostly the decision to eliminate base play, i.e. gens/turrets/invs mean basically nothing because you spawn in your load out. Also, the jetting/skiing physics are kinda wonky, in ways that I think negatively impacted gameplay.

The usual performance problems with tribes games, also the usual unintended difficulty increases by trying to make it "easier".