I think for a lot of people, it became the series you use to get your friends to try watching anime because it's super grounded and realistic compared to a lot of shows. It became the big classic series that everyone's seen, kind of like how Harry Potter is seen as kind of a classic fantasy series even though it's not really all that great. They're popular for being popular.
Death Note also really benefits by having a great hook at the start. Generally anyone who watches the first two episodes will see L tricking Light with that first tv broadcast, and be so blown away that they're hooked for the whole series. People tend to remember how smart the first 6 or 7 episodes are, and forget that the quality drops off pretty steadily after that.