Is it because of the large number of altcoins?
That is not the reason,every crypto currency is falling now so number of crypto currencies is not a matter.But normally the crypto currency price will hit down trend in beginning of year so this is happening in this year also but many factors causing the price fall is rumors and ad bans from reputed companies.
right . Once someone deliberately launches rumors to market, then people fear and sell what they have at low prices and this is the opportunity for those who create Rumors can buy bitcoin at low prices.