I think there are more than one cause of it
Symbolic logic is still a reasonably new field.
(Different than you may think symbolic logic didn't start with the old Greeks but with Frege's Begriffsschrift in 1879 , not even 150 years ago, and don't even try to follow his notation)
Some philosophers thought that they new everything about logic allready and didn't even study it and thus were never confronted with the standard notation.
Some logicians needed an other kind of implication (relevant, strict, material ) negation (minimal, sub minimal, constructive) or entailment (standard , fuzzy, quasi , degree) for their own logic and created there own new symbol for it.
Some logicians were comparing different logics and decided to use a different set of connectives to not get utterly confused.
A couple started their own notation because they were not satisfied with the old one. (Polish notation, dot notation, compressed dot notation, Lambda notation )
and maybe some wanted to confuse everybody :)
To add a bit:
even with truthtables you see these things:
- In some publications $0$ stands for true
- In other publications $1$ stands for true.
And that is just with two valued logic.
If you are lucky you have a book that uses $T$ and $F$ or $T$ and $\bot$.
In either case the $T$ stands for true, and the $F$ or $\bot$ for false.
But even so be warned , always check the meaning first.