Lyrics, Smearyics!!! I've recorded completely instrumental works of art with incredible guitarists and drummers that blaze thru melodies with sensual undertones and speak arrangements with strings in 5 different languages. Our instruments are our voices! Speak Load!!
Smooth when you take away the lyrics you narrow your target audience, theres nothing wrong with that if that is waht you want. But very few Instrumentals break thru to the mainstream, In the last 20 yrs Satch had that song Summer Song and Eric Johnson had Cliffs Of Dover. Im sure there are more but they are few and far between.
That being said as a guitarist I love instrumental fusion type music, but you arent going to hear it on the radio.