This is the reason:
DON’T USE SHAPESHIFT. Conversion is not the best and you pay the BTC transaction fee (witch is min 0.001 ATM).
So using shapeshift has this transaction fee and also from jaxx BTC to coinbase BTC you pay the same fee (min 9.5$ in tx fee)!
Best solution in you case is to use a trading service (like bittrex, kraken, bla bla) send zec there, make exchanges with placing orders (no fees) and then from your exchange send to coinbase and cashout.
EVEN THOO, I insist on informing that best way to cash out to FIAT is with ripple! Using ripple to cash out you don’t pay the BTC high tx fee and services like gatehub offer really honest exchange (XRP → USD/EURO) and SEPA bank transfers are 1 euro or internaltional wire is at 0.1%! That’s like almost not paying anything in cashing out.
KEEP IN MIND → any ripple account/wallet must have a minimum of 25 XRP as a reserve (more info: https://ripple.com/build/reserves/#base-reserve-and-owner-reserve), and there is a 0.2% ripple fee on every transaction (every every?).