The model works better for bundles of indie games which are frequently smaller, have lower price tags, less visibility, and offer more diverse experiences. But the supply of quality indie games is limited. Using larger games from publishers lowers the value of curation and probably raises the amount of money the publishers want.
The subscription model is an obvious trap to get you to spend more money on games you don’t need.They can’t afford the appearance of a trap, because Humble has little to offer over GoG or Steam, and goodwill and decent curation are the only thing that make them attractive as a shop. To avoid that appearance, they have a drop in, drop out model. Unfortunately, traps don’t work that well when you can just, you know, leave them. This is why most subscriptions have you purchase a couple months at a time.
From a customer’s perspective, I also find it less attractive. The subscription model is again, obviously a trap. I have too little free time to spend $15 a month on five games, one or two of which I will play much longer than a month and the rest I won’t use. I can frequently just actually buy the games I want or substitutes by themselves for a lower price.