There's no hardware or firmware reason that 2TB wouldn't work, but the 970 Evo itself may be an issue.  Read on.  The specs likely say 1TB because that's the largest SSD that Dell actually tested internally.  You'll find specs for several systems that quote a maximum capacity where users have successfully tested higher capacity storage once it became available.  The last time drive capacity had a real limitation was in the Windows XP days when you needed XP SP1 and an updated BIOS to use drives larger than (I think) 137GB in order to support 48-bit LBA addressing.  There's also 2TB limit on the Windows boot disk if you're using Legacy BIOS rather than UEFI booting, but your system likely uses UEFI anyway.

The potential issue with the 970 Evo specifically is that there are multiple threads on this forum where people have found that certain Dell systems don't work with it, even though the same system works fine with the 960 Evo.  If memory serves, I've seen these issues reported on a few XPS 13 and 15 models, and I don't remember anything either way for Alienware systems, but this is just an fyi.  You might want to do some research about the 970 Evo specifically on the Alienware 17 R5.  If you can find anyone who's used it successfully on that system, even a lower capacity version, then you should be good to go.