It's no secret that BTC has a big problem with fees and transaction times and it's getting worse with rising valuation and number of users. That's the first issue. Bitcoin's closest market cap competitors don't have these issues....at least not yet.
The second issue is that other cryptos have given better returns year-on-year vs. Bitcoin...especially in the past week to a month. During that period BTC went up 100% but some of its closest competitors in market cap have returned 200-300%....or better.
I am sure that majority of the coins will be having scalability issue in the future when there are as many users there are with bitcoin ,with rising value of bitcoin it is inevitable to pay higher prices for transactions and that is how things work here,and most of the currencies went up because the price of bitcoin sky rocketed and there are some coins which had their rally in the meantime like IOTA.