I drew the short straw! That's the short answer...
I was a great rhythm guitarist! I knew all the right chords, and could play rhythm guitar and sing the lead - though I did have some difficulty remembering lyrics...
I was a singer/songwriter/guitarist, initially. Then The Beatles came along! My rhythm guitar and vocals fit right in! However, rhythm guitar was a losing proposition... No one seemed to care about it... And, worse, bassists, when you could find 'em, were typically awful! And the good players frequently didn't have a working amp, or car (or ride when under 16 yrs of age)...
Most "non-singing" guitarist wanted to be a lead guitarist. Most singers could barely play guitar, let alone play bass! So I thought I had it made!
My first full time professional band had 3 guitarists, including me, and a pianist. One a VERY good lead guitarist, well versed in Blues, Rock AND Jazz! A real student of the instrument! He was also a very good singer! How rare!!! He was also the final player to join my soon-to-be-traveling band. The other guitarist (besides myself), also a very good singer, was quite good at finger style folk, and other fancy rhythm techniques we never utilized... He had 2 very expensive acoustics... All I had was a '67 SG... BUT, I also had this old Hagstrom bass lying around... Hence, I drew the short straw. Like Sir Macca himself, I was (still am) the reluctant bassist. But I took a page outta his songbook, and became a very good and steady - in demand - bassist!
I'm still a singer/songwriter/guitarist, as well as keyboardist, drummist, engineer, producer, and first rate jerk, when needs must! The biggest difference between now and then is that I have cultivated my multi-instrumentalist talents so that I'm at the very least better than acceptable. However, I must say, reluctantly, I've also become a very good bassist!