Why do the satellite manufacturing companies themselves make their crystals?
Space is tough? 

there's more radiation in space?
things in space can get very hot and very cold?

Satellites are expensive to launch so must last long?

2ppm drift over 15 years?



The Astrium website says


  Astrium Frequency Control Products group has an end-to-end capability in the area of quartz-based timing and frequency control solutions. This includes growing the highest purity quartz in the world, which results in the smallest radiation-induced frequency changes.
  
  The Frequency Control Product line ranges from simple VCXO and TCXO oscillators to Master Reference Oscillator Equipments
  
  The Voltage Controlled Crystal Oscillator (VCXO) is a quartz-controlled oscillator for space applications. The oscillator provides a mission frequency stability of less than ±30 ppm and a trim range of ±50ppm, plus low phase noise, low mass and low power consumption.
  The VCXO uses an AT-cut resonator fabricated from Astrium’s High Purity Quartz. The higher output frequencies are achieved by the use of internal frequency multiplication.
  
  The VCXO provides a compact, low power and low cost oscillator with a trim range adequate for many applications. This oscillator can be used in transponders and equipments where the oscillator requires to be locked to a reference signal for long-term stability.


e.g.