Why do people say you have to move laterally if you want to stay in the Marine Corps longer?

A litte background into my Marine Corps Career the Mos I started in when I tried to re enlist while deployed in Afghan the Career Retention Specialist slept on my package for nearly a month. When he finally got around to it(Msgt chewing his ass) my MOS was at 110% and I couldn't re enlist into that MOS so I had to choose a different MOS I chose a MOS where the Marine Corps is phasing out the platform that we work on. So I would have to move to something else all together, like everyone else said it is really dependent on the MOS.

So many of my Marines would hit HYT and get out as terminal lances because they just couldn’t get promoted. Lat moving to a MOS with a lower cutting score would, obviously, make it easier to promote and, therefore, keep you in the Marine Corps longer.

Use the troops trap? That reminds me of Army command sergeants major, with plenty of time on their hands, looking for minor things to complain about like you kind of crossed his lawn or grass or forgetting to put your troops trap on your open top top heavy 5 ton truck that would probably flip over if they hit a minor curb. How will the safety strap help you on a roll over? LOL