There were victims of Madoffs who YEARS after he was arrested believed it was all some misunderstanding or clerical issue. Some thought it was some govt conspiracy to take Madoff down and their funds were simply collateral damage. All kinds of fantastical (and highly implausible) conspiracy theories. There are people still TODAY asking where is the money. Even when explained that the incoming money was paying out prior investors for decades (and a nice cut skimmed off the top by Madoff) they hold out hope that the money still "exists". While they can comprehend that Madoff was a theif/scam artist they still cling to the paper totals and can't accept that there is not some hidden bank account somewhere with all the money.
As to your direct question. A bunch of reasons:
1) Some as simply in denial. If the believe it wasn't a ponzi they will get repaid and have the last laugh
2) Some accept they will never get the money but wont admit it is a ponzi because they will look stupid. So as long as they "demand proof it is a ponzi" well then they can win some kind of stalemate on the level of "nobody will ever really knows if it was a ponzi or not".
3) Some are criminally complicit. They knew it was a ponzi and encouraged others so they could profit off it. They will NEVER admit it was a ponzi. Hell if Pirate said it was a ponzi they would still admit they didn't know and come up with fantastical stories on how it *could* have been a non-ponzi.
4) Some simply want to be right. The ponzi vs ponzi "forum war" was divisive. Some people just want/need to be right even when they ended up on the losing side.
You will never have 100% of people admit something even when it is obviously true. Hell there are people who won't accept that the Holocaust ever happened. There are mass graves, stories told by millions of survivors, documents, news reports, the actual concentration camps still existing and some people STILL (and NEVER will) believe it happened. There will be far less direct and physical evidence of Pirate's crimes.
R-E-S-T-E-C-P ! (http://www.youtube.com/watch?v=klz6IVHfHpY)
but seriously, QTF the best post in this thread.