Why are plans only one paragraph these days versus the plank style?

Mainly because most of the planks are really not necessary. "Legislative intent" isn't really needed to explain that the affirmative gets to define their plan--that is now a given (well, unless the affirmative really shifts/has odd definitions of what the plan appears to mean).

The paragraph style takes less time read.

Also, the paragraph style focuses more on the policy action of the plan--rather than the board, funding, and enforcement, which are rarely issues in most contemporary policy debates (well, at least on the national circuit, college, and most regions).