Why wouldn't Mary Kate and Ashley join the show that practically started their careers for only 13 episodes?

Because UNLIKE the other washed up, has-been Actors who have probably all spent their fortunes and are now living in trailer parks, praying to to GOD for a reunion show for that last paycheck.................

the twins went on to form their own company, called Dualstar......which now makes BILLIONS .

Not Millions.............BILLIONS. They have a hand in a dozen different product lines, production companies. and so forth. They are both insanely rich.

Quite simply........they don't NEED to do the dog and pony show for the piddling paycheck.

They're both MILLIONAIRES , many times over.........

Ashley Olsen had stated that she hadn't acted in over a decade and therefore wouldn't feel comfortable acting again which is why she didn't want to do the show and while Mary Kate was willing to she had stated that it was just bad timing for her because of their fashion empire business.

they said they are to busy with their fashion line

Like i said in another question. They were literally babies on the show. They don't have the same memories as the other cast members do. They probably don't even remember half of anything. They were pretty much pushed into the acting business
They quit acting because they didn't enjoy it anymore. They have their own successful fashion line so it's not like they need the money.
It's not like they aren't thankful for the show either .They do see some of the cast time to time.

Maybe because they are now doing other things they'd rather do, or those other things take up all their time.

HAHA cause they can do whatever they want