Sharky and Kup, they're not doing too differently at all - with the respective issue #2s for example, MTMTE only sold 16 more copies than RID. And certainly by only issue #5 they wouldn't have that information in time to change the story, art and solicitations in time. It's a five-issue arc because it was written as a five-issue arc. Likewise, despite being an ongoing, MTMTE's first arc ends in #5 after a climax, with a resolution neatly tying up Ratchet's story. This too is so the TPB (Death of Op + MTMTE #1-5) has a strong and clear ending.
The most recent statistics on TF comic sales were:
149th bestselling comic for the month - TRANSFORMERS MORE THAN MEETS EYE ONGOING #4. 12,053 copies
150th - TRANSFORMERS ROBOTS IN DISGUISE ONGOING #4. 11,896 copies.