Born from math, bitcoin is a game of numbers.
Transactions have a size of ~500 bytes, blocks are limited to 1 MB and blocks can only be published after a puzzle is solved, a process that occurs via algorithm once every ~10 minutes.
The facts that underlie the current bitcoin protocol, set years before transactions could ever be called 'expensive', set the stage for the 'scaling debate' we've watched unfold ever since. Proposals have since developed to decrease transaction sizes, remove the block size limit and move transactions off of the main bitcoin blockchain, all in the name of boosting capacity.
http://www.coindesk.com/bitcoins-network-congestion-in-3-charts-transactions-blocks-fees/