They wish to prevent thw outflow of currency
Quite normal, there is no country that want money to flow out of the country.
Even goverments that have not banned but legalized usage of BTC are having the same policies.
They want to control the price of bitcoins
Once you ban it, it doesn't matter what the price is.
Many governments see it as a threat to their national banks
Apart from a few dictatorship states nobody really believe in this threat.
Many governments thinks bitcoins are made to defeat their economically big market that the posses.
I really don't understand what you wanted to say.
They think it will give rise to crimes and all
Every technology comes with a lot of crimes. This is the human nature.
Telephone scams, email scams, viruses every invetion was at least once used for crimes.
Due to decentralized in nature, if BITCOIN become popular then govt. will lost control on the economy. This will arise problem like illegal transaction, Money laundering, Drug abuse and Tax evasion. Therefore, govt want to ban it or regulate it under certain laws so that Economy could remain in their hands.
The goverment can always control the economy be it cash or bitcoins.
How do you think they are doing now and how do you think they managed durin the roman empire?
Tax evasion will be the same as it is now, you get paid in cash you get paid in bitcoins it's your choiche to take the risks and lie.