It's been a month since AskUbuntu is not accessible from Iran. And I am interested to know why? I can access it through Tor which is not very convenient.
Can you not use a VPN instead of Tor? I am currently in Thailand and many of the usual sites that I would visit are blocked since May (due to the Military take over). I tried Tor for a while but found that VPNs (PTPP, L2TP or (best of the lot) OpenVPN) work just as well, and are quicker and support BitTorrent.
I use Tunnelblick, for OpenVPN, on my MacBook.
Hope this helps.
P.S. I know that this doesn't actually answer the OP's question "And I would like to know why", but I wanted to add the VPN info as a helpful comment, but I am unable to as I am currently lacking in the reputation stakes at the moment.
Short answer: It's not blocked.
I'm a user from Iran since July 2014 and +5 month consecutive and I didn't see and don't see the askubuntu.com is blocked.