A better reason in hindsight isn't the right answer. Likely they had to do it per limitations of the computers of the time. Trying to tie it to UX like a modern day designer is just wrong.
It's probably not a good idea to underestimate the depth of the deliberations that actually took place at the time, based on a necessarily pragmatic justification for the accepted decision.
As discussed in the accepted answer, vertical line cursors predate the slanted shape.
Changing to a different style only makes sense as a design decision.