Because the BBC is a UK public service to provide TV to the UK. To do that to their best abilities they sell content to other countries/broadcasters around the world as the TV licence funding would not cover making all the programs they currently do and then just giving them to the entire world. If the BBC can't sell a program to any of the broadcasters in specific country because the broadcaster doesn't want to give it airtime there is not a lot they can do about that.
As far as sports coverage etc. The region restrictions are not something the BBC does off its own back but something that is imposed on them by the Events as the Event wants to sell rights for more money to countries where the broadcaster charges per subscription. If the BBC has to provide unrestricted access to the whole of Europe at the price the UK viewer pays (iPlayer is essentially a free service, paid for indirectly) they will in fact have to cover fewer events and they possibly won't be able to continue their iplayer service at all.