Because if you were most people would pick the tiger since it's the better tank. I rarely want a 20fav turd mobile (my 3AD deck had one in B for shoving through choke points and zero in C), but a 1.2km he slinging beast with max range panther penetrating AP at 2 stars in my 3AD deck? That's pants changing levels of good. Compared to the 76 Sherman it's a fucking monster.
Edit: my jumbo opinion might change now that I can reliably repel the HS129 with 3AD AA before it shoots. I just don't see much reason to bring it when I've got Shermans and smoke.
when using no tactic : tiger is better
when using tactic : jumbo is better
when fighting noob: tiger is better
when fighting skill: jumbo is better
jumbo thank to extreme armour has more battlefield influence... he can roll up and dumpster an entire portion of your flank while you avoid LoS Pak 43 or stun them with 3ADs great artillery. while a tiger if he wants to safely engage has to sit at max range and lob 5 HE shells (since a 6 pdr will wipe this out - dont get started on 17 pdr or 76mm), when a jumbo will move in and delete all infantry and AT guns with .50 cal and .30 cal MG, and faster firing 5 HE cannon. and once the tiger/panther is forced into close combat to deal with said jumbo the shoe is on the other foot thanks to stabilizer.
if u dont use jumbo in 3AD i really question your gameplay.... it is like BMPT if the BMPT could blast away tanks that cost more than itself at <600m combat.. it is power unit that forces your enemy to react because of how hard it is to deal with when play correctly: staying out of range battle, cleansing the map of anything short of cats and if u want to deal with it u drive your panther or tiger into a compromise position.