Because he goes through a solid amount of story development over the course of the series with his share of memorable moments. Garrus is just a beautifully handled character from a narrative standpoint.
Of course, none of that really matters compared to what happens near the end of ME3 where he...
[spoiler]Hooks up with Tali if neither of them are involved with Shepard. You go, tiger.[/spoiler]
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[spoiler]i hooked up with tali so i didnt even know that could happen, good to know. Tali was always my favourite of the female characters, which is why i was kind of annoyed you couldnt be with her in 1. [/spoiler]