As far as I understand, most modern CPUs and platforms are 64 bits, including mobile processors.
If I develop a new programming language it would be much easier to target only 64-bit platforms.
Maybe I don't know something and there is a huge market of 32-bit solutions, except legacy stuff?
It's depend of what your language is intended for. Most recent hardware are still able to run 32 bit app and OS. If you want to develop drivers or anything close to the OS, you will need it.
Also on ARM there is still a lot of CPU out there which doesn't support 64 bit at all. This announce the 64 bit ARM it's dated September 12, 2014...