I don't understand why people are saying "XRP can't go up because BTC is doing too well"

Do you not understand that XRP and BTC have almost nothing to do with one another?

Most altcoins' performance are in some way tied to the performance of BTC, but XRP does not fall into this category. It is a totally separate and independent blockchain. BTC's price rising or falling is completely irrelevant when looking at XRP.