(Answered by Douglas Cutler)
The answer is simple though you may not be willing to accept it.
But before I tell you I will suggest we could ask the same question of all the great aging singer-songwriters like Dylan, Joni Mitchell, Bruce Springsteen, Elton John, Neil Young, Jimmy Paige, Cheryl Crow, Leonard Cohen (now deceased), Gordon Lightfoot, Paul Simon, Billy Joel, even Sting. So many more. While many of these writers continue to write really good songs, they virtually never write material equal to the A-list songs of their prime. Why?
Also, ask why younger songwriters just don’t seem to quite measure up melodically to their musical forefathers. Also ask why we never hear new instrumental pop hits anymore like Linus and Lucy, Green Onions, Classical Gas and Cast Your Fate to the Wind. Also ask why we never hear any more great new tunes in the American Songbook style – though thousands of musicians try every year. Also ask why classic country and western seemed destined to morph into new country.
Same answer in all cases: we’re running out of melodies. Melody is a finite set made up of only 12 steps in the chromatic scale, except in most cases 8 notes of the diatonic scale or 5 to 6 notes of the pentatonic scale which dominates so much soul and gospel flavored pop. Also a gospel and blues connection to country music. Plus, many musical motives define themselves in as few as 4–8 events. Think of the opening to Beethoven’s 5th: Ba Da Da Dah. Plus, there are further limitations like a melody must be comprised primarily of easy to sing step and skip-wise motions. Mathematically you could argue the possibilities for 12 pitches still run into the billions but most of those are random combinations full of hard to sing chromatic jumps. Also billions of atonal combinations but who cares about those?
Also, many melodies just sound common and mundane even if they follow the rules. Writing a hit song is like writing out a sequence of numbers equal to a winning lottery ticket, actually very rare compared to the hundreds of thousand songs that get written every year. There is something magical about a certain combination of notes that stick in your head in a uniquely pleasant way but such melodies just seem to get rarer as the years go by.
Break it down, Linus and Lucy melody in solfege: Do-Re-Mi/ Mi-Re-Do/ Re Do/ Do-Re-Mi-Mi. Can’t get much simpler than that and once it’s written it gone from the possibility set for the next generation of writers.
Every Moment Has A Song 