It's not a tower defense clone. They added some constraints (no lateral movement) and removed others (no upgrades, set list of "towers" per game) which makes it more addictive.

They've obviously spent a lot of work tuning the game fundamentals and then added a great cute theme/graphics.

I think you are missing how important details are. This is why game designers for both board and video games spend months tuning their game mechanics to get it just so.

This is why Sid Meier games have his name on them and tend to be very popular.