Many leading Linux, such as Ubuntu, Arch Linux, Manjaro, etc., have already dropped support for 32-bit hardware and decided to focus on 64-bit machines. Now, following their example, Ubuntu Budgie (source) and Ubuntu MATE (source) have also dropped support.
The main reason behind these changes is the decreasing number of users running these operating systems on 32-bit machines. So it's obvious that it doesn't make sense to continue to allocate resources and time to hardware that people don't use anymore.
The flagship Ubuntu decided to make this change starting with 17.10. Ubuntu Budgie and MATE will do the same starting with the next release, 18.10.
The team behind MATE says this change will save time and provide better support for ARM like the Raspberry Pi. Development teams also have limited access to 32-bit hardware, which doesn't ensure tight controls.
What if I still want a 32-bit Linux distribution?
Don't worry. If you still prefer your older hardware and want to get some work done, you're covered for the next 3 years. As this change will take effect in 18.10, 32-bit 18.04 will continue to be supported for the next 3 years, until April 2021.
