Lisp is not a single language, but a whole ecosystem of different languages. Moreover, there's no standard covering all Lisps, like with C or Fortran, so for this reason,
plus are equally "valid".
When Lisp 1 (March 1960) was written, the primitive operations defined were
cond, etc. The arithmetic operations were not primitives at that time, so the programmers chose their own names.
At least Lisp 1.5 (early 60s) had both.
But this Lisp from 1970 had
MINUS but no
If you consider Scheme (1975) to be a Lisp, then it is a specimen having both
&+ (the latter is an optimisation for two arguments only).
And Common Lisp (1984) has
+ but not
plus as you have noted.
So I posit that we gradually settled on
+-style symbols starting in the 70s, and the situation was a state of flux before then, for the reason that arithmetic operations were not even primitive operations to begin with.