Possibly a long time, MTGox could have paid out years ago if they had have paid the value in Yen of bitcoin balances at the time of collapse. They could have paid everyone in full and had some billion left over. This would have made a lot of people very unhappy.
As the value of bitcoin went up it was clear a better solution was needed, and ultimately one of those solutions was decided upon then put to a vote.
At each stage they were required to go slowly to go slowly to give people time to respond. People needed to be given a timeframe to make claims, verify their identity, vote etc.
On top of that there were lawsuits (as there inevitably will be when someone is deciding how to divide up billions).
Add to that plain old bureaucracy and managing paperwork of thousands of claimants.
For the record MTGox had a deadline of something like October last year, and was given a 1 year extension on that by the courts. I'd say 50% chance they meet that and pay out this year and if not they get another extension and probably next year.
Even after that payout they would not be entirely done, they have set aside a small (proportionally, probably huge in dollar terms) sum to deal with accounts that are in dispute. That'll last as lawsuits can last. The 'early' payment is for people who opted to forgo their portion of the disputed amount, which they might or might not have otherwise received.
So yeah, it takes a long time, but because of very real complexity and dealing with people with differing opinions.