Why would I want to keep BTC in my investment portfolio? That's most hyped and quite overvalued coin I'd say.
There are other more technologically advanced coins. Is it because of "digital gold" and store of value properties?
The main reason why most of the investors are still holding their bitcoin is because it is not the right time to sell bitcoin. If you can see the bitcoins value, the value reach the red point or simple it drops it value. So to avoid losses better to hold and wait for the right time to sell. Personally, I hold bitcoin because I have nothing to do with it for now, if I sell it maybe it will be spend in not so important things.