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.
Yeah, that's not the one factor that bitcoin price was down these days there's no effect even how many altcoins in the market bitcoin will still remain. Banning of ads in social media's maybe one factor of these happenings now the Google, Facebook and recently Twitter, these 3 reputed companies now are deleting ads regarding those users who post and share in ICO's project.
Meanwhile, this may affect also bitcoin price in the market that Japan now some exchanges are closing and one of these is Binance.
But that's only a speculation we all know both that bitcoin is decentralized coin maybe also it is a part of bitcoin that have a volatile to get tempted those investors to invest when price almost hit the floor.