Voltage divisor on a oscillator output using Capacitors vs Resistors
XTAL_IN on the destination device may have very little scope to deal with slight offsets of its natural DC operating level. So, if you were to use a resistor divider you may well have to add a third component, namely a capacitor between the divided down signal and XTAL_IN.

It's likely that the oscillator pins on the destination device uses a high value resistor between OUT and IN to naturally set the optimum DC bias point at IN. This is because internally it is likely to be a CMOS inverter made to operate linearly using typically a 1 Mohm feedback resistor.

The other issue is the XTAL_IN input capacitance. It's likely to be several pico-farad and if you used a resistor divider, the natural input capacitance would create a phase error and may even attenuate the resistor-divided signal so much that it just won't work. 

Using a capacitor divider works on both issues mentioned above.

This is typical of what might be seen behind the pins XTAL_IN and OUT: -