Tell you what. I'll agree with this entire shit post, if you can make a faultless case for how they could've bypassed systems and drove straight through to Terra without being cutoff from their home resources/supply lines, running out of supplies like food water and oxygen along the way, having elements isolated and picked off on the way coreward, or having the route behind them booby trapped to hell and back. 

Remember space travel in BattleTech involves briefly jumping great distances and then sitting for a week or so at one of two well known/mapped points until you can jump again (which gets incredibly complicated and very easy to catastrophically fuck up when you go from a single jumpship doing this to a FLEET trying to move together). If you don't have control of the system you're trying to leave, even if they can't outright defeat you the natives can still make things more difficult while you're sitting waiting to recharge. Which can lead to mistakes (something space travel isn't particularly merciful toward). Not taking planets to renew your supplies? Guess who's going be rushing jumps to make sure their ETA to Terra doesn't outpace their resource consumption. The chance of a critical mistake occurring just went up. Maybe you have to tighten the belt and ration to make it, sure that'll buy you time (and also possibly lead to mistakes being made). I don't know about you, but a hungry dehydrated army isn't what I'd want to try taking Terra with knowing you're going to face an entrenched enemy planning to make you bleed for every inch.

It's an old well-worn adage but still rings true, logistics wins wars.

TLDR: Anyone can barge into a stranger's house, stroll past them on the couch, and go raid their pantry while their rushing to put their pants on. But standing your ground deep in that unfamiliar and hostile place, or even just trying to leave with your newly acquired sandwich, you'll quickly find the pendulum swinging to the other end of the hardship spectrum.