ORA 1719 outer join operator (+) not allowed in operand of OR or IN

I have recently encountered this error in a query and am trying to understand why Oracle doesn't allow this.

For example, the following yields the error:

select A.col1, A.col2, B.col1, B.col2
from table1 A, table2 B
where A.col1 = B.col1 (+)

    and (B.col5 (+) = 'X' OR B.col6 (+) = 'Y')

However, this equivalent query with the "or" condition expressed in two separate selects which are then unioned, works fine:

select A.col1, A.col2, B.col1, B.col2
  from table1 A, table2 B
  where A.col1 = B.col1 (+)
  and B.col5 = 'X'
  union
  select A.col1, A.col2, B.col1, B.col2
  from table1 A, table2 B
  where A.col1 = B.col1 (+)
  and B.col6 = 'Y';

My query is now running but -- any ideas on *why* Oracle is requiring this join to be expressed through two unioned selects, rather than just one select?

--
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   Jay Canha