There are thousands of cryptocurrencies available in the market. Every now and then you will see a new ICO . At this stage bitcoin is around $163,037,447,966   worth market capital. It is not that easy to crash a currency worth that much and spread all over  world. Sometimes crashing a company or currency is somewhat easy compared to the entity spread across multiple continents.
Even if someone tries to crash the bitcoin in US or europe, markets from other part of world will happily cash in for the opportunity.