Why do the 3-Dimensional Sonic games suck so much

First of all, the idea of something "sucking" is merely an opinion; some people may agree that the 3D Sonic games are poorly made, while others may not.

In general, most of the 3D Sonic games were met with poor reception because of the gimmicky features and large number of new characters each one added. Additionally many previous fans felt that the games, in 3D form had lost much of what had originally attracted them to the Sonic series in the first place. With all of these complaints on top of the apparently small amount of time that went into polishing the game, the games were usually rightfully disliked for being of bad quality.