The video game industry has a weird relationship with sequels. When something becomes popular, the publisher behind it will waste no time in putting hundreds of employees to work to ensure a new project is released regularly, which has given us a world where players get a new Call of Duty every single holiday like clockwork.
As a result, more often than not you can guarantee that a blockbuster game will get a sequel at some point, but the problem comes as soon as the franchise falters even a little bit. As quick as they are to put a series on a conveyer belt production line, publishers are just as quick to guillotine it as soon as sales numbers come in under expectations.
As a result, there are a bunch of games from across the years players have invested in only for their time with them to be cut short, as anticipated follow ups are shelved.
And no, Half-Life 3 won't be on this list. I'm sick of it, you're sick of it, let's just acknowledge it here and move on with our lives.