It's partly that: creating a good A.I. is more difficult than creating good-looking graphics, and there are more skilled artists than A.I. programmers. Both of them are resource intensive, so it's hard to have both will keeping system requirements down. The poor A.I. from R:TW may well have come from the decision to have only slightly higher system requirement compared to M:TW, while massively improving the graphics.
However, it is mainly because good-looking games sell better. The mainstream player is not going to play the game for long and doesn't like steep learning curves, so he prefers graphics to a really advanced A.I. It also generally takes a week or so of playing for the exploits in the A.I. to arise, so reviewers, especially if they are not very familiar with the game and it's community, don't initially catch on. Also, graphics are by their nature very visible in the advertising; a fantastic A.I. on the other hand is at best a bullet-point.
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