I've read that there was some type of dispute between a french rabbi and a lot of European ones in the 1900s (http://www.icar.org.il/conditional-marriage.html) so I was wondering how could anyone say its assur if it says in kesubos 73a and b that a condition can continue from eirusin through the rest of marriage?
The reason is simple - we don't strictly follow the Gemmora if the later Poskim (Gaonim, Rishonim, Achronim) say otherwise. The examples are numerous.
Now some discussion on the topic: I understand your question is about avoiding Aginut (or maybe Mamzerut also) by applying a condition that will annul a marriage retrospectively. For example - "You are Mekudeshet if you won't cheat on me", or "You're Mekudeshet if I don't loose contact with you for more that a year". Once the condition is violated, the marriage will be withdrawn retrospectively.
I was sitting on this Sugya for about a month with my Chevruta. I'm bad citing names but most Rishonim and Achronim can not agree on a couple of basic definitions. The main being the meaning of "כל המקדש אדעתא דרבנן מקדש".
Some claim that דרבנן means only the Gemmora, and what's not ruled there we can not change. Others claim every Beis Din has the power of setting its own conditions (e.g. in medieval Europe - I don't remember who mentions that) it was accepted that a marriage will not be held (i.g. valid) in less than a Minyan.
Another problem is the Kesubah - if the marriage is annulled - there's no obligation for paying Kesubah. Poskim disagree on this also.
Another problem is the legal status of their previous saxual relations with Kiddushin annulled - was it Znus, was she a Pilegesh, is she now Muteret to a Cohen etc. Poskim disagree widely on this also.
Another problem is it is not clear whether the couple can have Nissuin (relations) while Kiddushin were on condition. Poskim disagree on this also, so there's a serious problem of Yihud in conditional marriage.
Another problem is that this opens a wild opportunity of transgressing, knowing that nothing serious will follow. A man can disappear, a woman can cheat etc - we're not so sure we like this option.
Another problem is whether an active decision of a respectable Beis Din is needed to actually annul a conditional marriage or it annuls itself, like there was no marriage in the first place. It is not so clear from the Gemmorah and the Poskim disagree.
The biggest problem (in my opinion) is that although we have numerous examples of stipulations from the Gemmorah to Shulkhan Aruch, it was never practiced. I could not find a single Posek that arranges or witnesses a conditional marriage. We would all love to find a better solution to numerous problems (e.g. Olim couples from secular countries that come to Israel have an unclear status etc) but we don't have a tradition for suddenly conditioning a marriage.
Hope this will help to understand the Sugya.