1. It will be difficult, and expensive, to replace it when the tank fails
2. If you do not put a drain pan under it, it will cause extreme damage to the house.
3. If you do put a drain pan under it and ITS drain is plugged, or the heater leaks faster than the pan can drain, it will still cause damage to the house.
45 gallon is an odd size for a water heater so is it gas or electric. If it is gas then providing a connection for the heater in the attic could be difficult, depending on how the gas pipe is installed.