Easy (also known as Casual in Guitar Hero Live) is the second least challenging difficulty in the Guitar Hero series, after Beginner. It only uses the green, red, and yellow fret buttons. This eliminates moving your fingers up and down the frets completely. The fret board also moves very slowly in easy. This is the difficulty that most new Guitar Hero players start on.
People who play Expert might have a hard time because they aren't used to it, seeing how Hard and Expert difficulties require rapid hand movement and finger dexterity, whereas easy requires more precision due to it being so much slower (a theory proven in the Guitar Hero III: Legends of Rock tutorial when The God of Rock and Lou are arguing about a guitar battle. Lou mockingly asks, "What's it gonna be this time; 'Hit Me with Your Best Shot' on Easy?" to which The God of Rock hesitantly replies, "N-no! I've been practicing!" implying that had trouble adjusting to Easy mode's slower pace at one point in time. Then The God of Rock suggests heading to Mt. Olympus for some "Expert DragonForce," implying that he can play 'Through the Fire and Flames' on Expert with relative ease.) This is why some more experienced players choose to use hyperspeed to full combo songs on easy.
Gameplay Elements that Make Easy harder than Beginner (GH World Tour Only)
Beginner only requires open strumming and no notes, while Easy actually required the Green, Red, and Yellow fret buttons. Also, both levels move at the same rate. So essentially the major difference in all of this is the use of notes and fret buttons in general.