Because they had to pay goodreads a commission each time (say 5% —> i’m completely making this number up and its probably a lot lower).

So if 70% of goodreads profit is from Amazon, amazon figured they could just buy them since the cost of running goodreads is a lot lower than the commissions it makes so amazon was basically paying them anyway. better (in their view at the time) to just pay goodreads one off and buy the company than to pay each year in perpuity. depends on the price they bought it at