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.
You are partly right, BTC and ripple has very little in common from tech view.
But the altcoin market is very sophisticated .
The situation when btc is going rapidly up but ripple is not could be explained with scenario when traders are buying btc with funds from selling other alts in short term because of impressive btc gains. Thus the statement "XRP can't go up because BTC is doing too well" could be true.