One engine block (the taller "RG" item) instead of two (the LG 170 and the RG 225); creating a small basic-equipment engine using only different crank and rods and carburetor. Less expensive to produce that way, and by '70 the A-bodies were growing heavier and rear axle ratios were getting taller and the 170 had gone from underpowered to severely underpowered, so the 28 additional cubic inches helped.
Why was it made for only 2 years? Did it absolutely suck?
No, it didn't suck. But at the same time as cheap-and-nasty emission control strategies were taking a big chunk out of engine performance and primitive safety engineering was weighing the cars down heavily, putting the 198 in the same position as the 170 in 1969, Chrysler was running low on money and realized it would cost even less to make just one 6-cylinder engine rather than two. Add to that the forthcoming-at-the-time changeover to a cast crankshaft and the forthcoming-at-the-time 2bbl package for the 225.