Is there any other reason for this other that the high risk and high volatility of altcoins compared to bitcoin?
Being a high risk alone is already enough to become skeptical on any thing especially on investment. Also statistically wise, Almost the majority of altcoin is useless or turn into scam and only few has real use case that can generate profit for project sustainability.
Its not bad to invest in altcoin if you knew what you are entering. I believe the reason behind why most people suggest to invest only on Bitcoin here is because this is Bitcointalk and people preferred Bitcoin due to its feature.