Because yahoo is now retiring Yahoo Messenger version 6.0 – 7.5 ..
This is according to Yahoo Messenger Blog..

It is no longer able to login to yahoo via any client that is using Yahoo protocol below than 8.0

hehehe.. and so does the pidgin 2.5.5
about-pidgin
I don’t know if Pidgin 2.5.6 already supported Y.M 8.0 protocol..

It is happening since this morning.. yesterday still can..

So, option that we have now are :

1. Use yahoo web messenger.

2. Use built-in chat .. something like gmail in yahoo mail web interface;

3. Install a client that support Yahoo messenger protocol 8 .
such as gyachE. (I don’t know why it is spelled like that.. 8-))
gyache-help
installation step is available in my previous post at How to install Yahoo Messenger with 8.0 support in Linux

And after done with installation. during login screen of gyachE can set the protocol to use to go online with yahoo :

gyache-option

Kindly select the latest protocol.. else you wouldn’t able to login to yahoo..

Hmm.. some reason that i can think why they stopped older protocol is
1. because of the spammer tend to abuse and send massive junk message to user..
2. security concern over the booter/flooder user attempt to hurt each other.
3. stop and fix flaws in previous protocol.
4. push all other client to move forward with latest protocol. (wonder why pidgin can’t support many additional feature ~ stuck with some 3year ago technology)
5. encourage usage of built-in chat box in Yahoo mail.. now can popped up to the bottom right of the browser also.
something like gmail used ..instead of one page just for chat only before this. (one way to announce new feature in Yahoo Mail..perhaps)

p/s : now realized..~ that pidgin are now also outdated for Yahoo usage.. or could be because developer too busy to migrate it to use new protocol.. 8-)

Luckily got many other communication transport already established : .. “Google talk, Facebook, Twitter,ident.ca , MSN,Skype..”
So .. a bit less damages .. unless for some “long-time-no-see” friend which only connected via yahoo.!