Because at the moment, bitcoin has no redeeming qualities.
- it is not transactional, as each trade costs $30-40. as exchanges hold the orders for weeks
- it is not rare, as there are tons of viable cryptocurrencies coming out each day
- it is not safe, as exchanges go bankrupt and shutdown.
- it is not stable, as prices fluctuate wildly each day. Downside could be 90%, upside could be 20%.
- it is not fast, as 100K+ transactions awaits
- it is not anonymous
- it is not valued correctly, as a sell order of $100M will most likely reduce the price by 50%