I don't think there is really a need to be afraid of 51% attack on small altcoins, as usually the attack will require the attacker to put a lot of effort a very big invesment, which he usually won't be able to recover because after the attack will be completed he won't have enough liquidity in the market to liquidate his amount and sell his profits.

The fact that altcoin's markets are usually really small just make it very unprofitable for people to attack because the market will drop soon after the attack - lack of liquidity.
51% is no more profitable- but it is being done by people who want the hurt the specific altcoin- usually an altcoin that is attempting to steal an idea- they will attack it in order to make their altcoin raise.

I do not know why so few such attacks have occurred so far, because the attacker, in fact, does not risk anything, you need to purchase equipment, conduct an attack, after this attack the equipment will remain, it can be further used for mining.