Because you wanted to use the old Leenex 60 plants, and you've got enough- ahem- spare pilots to have a few.
Urbanprotomech
Ultraheavy Protomech
Mass: 12 tons
Internal structure: 30 total points - 1200 KG
Engine: 60 size fusion - 1500 KG
walking MP - 3
Running MP - 5
Jumping MP - 3
Jump jet weight - 450 KG
Cockpit, life support, sensors, pilot: 750 kg
Heat Sinks: 1 compact protomech - 250 kg
Armor factor:
Head: 3 (7)
Torso: 12 (11)
Arms: 3 (5)
Legs: 7 (11)
Main gun: 2 (6)
Weapons and equipment - Loc - Heat - Ammo - Mass
Protomech AC/4 - Main Gun - 0 - 12 - 5100 KG
Micro Pulse Laser - Torso - 1 - 0 - 500 KG
Originally I wanted to use the Protomech AC/8, but that weighed too much for me to have good armor or to keep it at 12 tons so it could use the 60 power plant. But having the chance for a point to force a PSR out to 15 hexes can't be laughed at.