Because that’s how things work. On the first few days, there’s a surge of players, trying it out and learning it. As people experience more, some find they don’t like the game that much after all and leave altogether, others turn to more casual playtime, playing occasionally, while only a small portion stays on constantly. Yes, troubles with bugs and other things might turn more people away, but others will just come on the forums to complain - or to try to make things better. Even after the first major player drops, people will trickle away. Some will come back for a while when major content gets released (1.1 in this case, and later DLCs), but even with a surge of a few new players, the total amount will drop slowly until it’s practically zero, or until it otherwise stabilizes to only the most dedicated players.
Looking at achievements’ global stats shows me that a quarter of players haven’t even equipped a Common item, less than that have reached even level 5 on one character, while less than a third have gotten Skittergate on Recruit. It gets only lower from that, only a fifth of players have gotten past Veteran. Considering what I remember seeing on other games, those feel somewhat high numbers, actually. All that informs us how much of that initial surge have really tried to play the game and gotten past the beginning stage. Other games have similar profiles. Comparing to one other game, Borderlands 2, another popular co-op game, 72.5% of players have completed the first achievement, gotten partway through the “tutorial” section. That’s less completion than this game’s equivalent, and (arguably) easier to get. Only slightly more than a quarter of players have even completed the story.
So, yes, losing players fast after the launch surge is very natural for a game, and nothing to wonder or be worried about. If anything, we are getting to a stabilized population now, getting smaller surges with new content and possible Free Weekends later. Several months more is needed to confirm that though.