My guess would be that they found D20 initiative rolls to have too big impact on battles, and thought it was easier to balance them with less randomness.
That may be true, but I've played alot of TT, and never thought d20 initiative was an issue. It would be interesting if the devs could explain their design decision for the initiative system.
I'm only speculating ofc. but I could imagine a computer game might benefit from being a bit more consistent in difficulty. It's easier for a live DM to tailor the encounters to the players' skill level and party composition, while a game dev has to create more of one size fits all solution. With the large number of players, even statistically unlikely scenarios such as all PCs ending up at either the top or the bottom of the initiative order are bound to happen for some people. And though some might enjoy the extra randomness, others might think it unfair if for example the enemies burst down half your party before you get a chance to do anything.
But if you're into modding, I'm pretty sure I saw a D20 initiative mod. Though it would be nice if it was at least available as a custom game option.