It's explained in the announcement. Perl 6 was already renamed to Raku, but calling the new Perl version Perl 6 would be confusing, so they are directly jumping

It's explained in the announcement. Perl 6 was already renamed to Raku, but calling the new Perl version Perl 6 would be confusing, so they are directly jumping to 7. They even show other examples of version jumps.