Currently, Bitcoin has a certain limit on the size of each block, hence this translates to a limit on the number of transactions per block. With an approximate 1 block every 10 minutes, this imposes a hard limit on the number of transactions per given period. The Bitcoin protocol will be changed eventually to increase this value, and it is suspected that by that time, the Internet bandwidth and storage capacity will have also increased, thus allowing for such change. This subject was brought in a conversation with Satoshi Nakamoto. Until then, we have this limitation which does not impact Bitcoin users for the moment as there are only 3 to 7 transactions per second right now, well under the limit.
http://bitcoinmagazine.com/17275/speeding-bitcoin-transactions/ 
