Halving is the main factor, if we look back in 2015 bitcoin marketcap is only USD 6.45 bn , 2016 is USD 15.49 bn, 2017 is USD 237 bn, 2018 is decreased gradually to USD 66.8 bn. I think altcoin isn't get stronger but bitcoin which weakening. Now halving makes people attention focus on bitcoin so it become stronger again.