I think this is because Blizzard/Activision is already a successful gaming company with many years of experience of player retention (via WoW) and deep pockets. Thus, they don't need to squeeze every penny out of a user now if they know those users will come back every time a new expansion is released to buy more cards.

That said, there are still players who complain about it being too hard to get into the game for new players, but I think Blizz is addressing those issues decently well.