Why did the ball point pen replace the fountain pen?
Many reasons:
Ball-point pens are simply more convenient. They don't leak and
the ink lasts for much longer. Ink replacement is almost
unnecessary because buying a replacement ballpoint pen is so cheap.
Ballpoint pens can be kept almost anywhere without fear that ink
will leak or dry out. Fountain pens, on the other hand, must be
constantly cleaned out and refilled.
Ballpoint pens are much cheaper to produce. They can be made
almost entirely of plastic, unlike fountain pens. Ballpoint pens
are also easier to mass-produce without having to craft nibs
separately.
While fountain pens are used for fancy writing and have overall
better ink quality, they take much longer to dry. This causes them
to smear easily and sometimes run down the paper, easily ruining
the work.
One last but less obvious reason is that since fountain pens are
designed to be pulled and not pushed across the paper, a
left-handed person cannot use one without wearing down the nib
faster and likely tearing the paper. That's because a left-handed
person rights from left-to-right, pushing the pen across the paper
as he writes. The current selections of left-handed fountain pens
are quite limited and uncommon to see.
As you can see, a ballpoint pen has none of the problems that a
fountain pen has, sacrificing a little fanciness and legibility.
Most people these days don't mind at all. After all, they're so
abundant that you can keep them everywhere.