One reason that comes to my mind is, that an UDP packet of size less than 1500 byte fits well into a single Ethernet frame (MTU usually 1500 byte) without IP fragmentation.Usually, UDP based protocols are suited for fast (unreliable) exchange of small information chunks (e.g. Radius, DNS, etc.). You can benefit from UDP in contrast to TCP because you dont need to execute the threeway handshake in advance. In case, you need to transfer only small datagrams that's faster with UDP. However, higher protocol layers may need to implement retransmission schemes and the like to ensure a better level of reliability. So I guess, the reason is avoidance of fragmentation. Hope that helps.
Update: If you can, do not use PDU-sizes bigger than 1500 bytes.