What's the evidence that the top 20 coins are easy to manipulate? It's mostly just people saying that.
Altcoins severely lack liquidity, both on the buy and sell side, and this gets worse the further down you go on the list. The lack of liquidity is what makes altcoins an easy target for manipulation by either Bitcoin whales or institutions milking their financial advantage in the market.
You surely have noticed how altcoins generally pump and dump harder than Bitcoin, and that on way lower volumes. Altcoins are almost leveraged instruments on their own, which is why people tend to park their funds in altcoins rather than Bitcoin during bull runs, and convert back to Bitcoin during corrections.
If you make sure there is an incentive for deep pockets to manipulate altcoin markets, which there is if you can either long or short their prices, the manipulation kicks in naturally, especially in case there is next to no liquidity preventing you from toying with certain altcoin markets.