I can't speak for why they made the changes they did, but things to note about the 3.0 versions of the buff spells.
a) no mental buff spells, eliminating spell DC inflation
b) the variable range meant that they could be maximised and empowered. Or since metamagic stacked in 3.0, multiple-empowered (so a wizard might prepare a 6th level double-empowered cats grace to get +4-10 on their Dex, a 9th level maximised double-empowered one would give +7-10. it was common for high level wizards to get a lot more mileage out of their buff spells in those days.
c) the possibility of an 'odd number' bonus benefited those who hadn't min-maxed their abilities to all be even numbers.