More power. The engines bigger than 12 horses needed the frame notched to fit the bigger flywheel. (Hence the 147) Rather than produce 2 separate frames, they updated to one larger frame that could fit all the engines, and while they were at it they made the driveline more accessible with the removable tunnel cover, so tractors didn't need to be split as often. NF cubs, though my favorite, can be a royal PITA to work on anything outside the engine bay.
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Ian
Mine: 72 w/ k301, 3 125's, 1 w/ hydro lift, 38,42, & 2 48 decks, 2 42 front blades, QA-36a & QA-42a thrower, tiller, lo-boy 154. Also, LOTS of parts.
family's:2 105's, 2 106's, 149, 2 lo-boy 154s, Farmall Cub.
Non-IH: 1940 JD Model A, 1954 JD 40 U, 1955 JD 40 Crawler, 2 1956 JD 420 U's, both w/ Henry Loader and Backhoe. JD 110. Wheel Horse (model unknown.) Power King 1614. We love our tractors! 
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