People are more inclined to enter the pot in position with drawing hands than open the pot with drawing hands, so if some one is flat calling behind you, they're likely flat calling with a hand that doesn't rely on high card strength. The most common of these hands are suited hands with some high card strength, like AXs and 20s and from there suited connectors. When one opponent flat calls, he could be on either a drawing hand or a pocket pair, but as other opponents flat call behind the first caller, the likely hood of them calling with a drawing hand is greater because the EV of the hand increases for each player that entered the pot before them and each player that enters the pot behind them.

Additional pocket pairs could be calling for sets behind the first two flat calls, or the drawing hand could have been the first flat caller (anticipating others would flat call behind him) and the set could have been the second flat caller, but the odds of two pocket pairs calling to sets are a lot less compared to the odds of two cards being suited or connecting, and the conditions for flat calling with drawing hands are more preferable with 2+ players in the pot already.

There's an effect called the "Schooling Effect" where as the fish limp or flat call with bad hands, there collective bad plays and bad hands have a domino effect that turns there bad plays and bad hands into good plays and good hands against some one who is relying on high card hand strength by creating implied odds. For instance, if you open UTG with AJu, and 9 other players call, your AJu is garbage even if it hits the high card. If you had opened with JTs, and 9 other players called, you'd be in great shape if it hits the draw. Similarly, if some one opens UTG and 8 other players call, you'd rather have JTs to call with and fold AJu.

Since high card strength weakens for each person in the pot, some one who enters the pot with a high card strength hand in EP needs to re-raise to isolate UTG, secure his position and prevent other players from entering the pot. For instance, if I'm holding AQu in EP and facing an UTG raise, I'm not only re-raising for the folding equity, I'm re-raising to price out other hands from entering the pot behind me. The same can be said for JJ, I'm not only re-raising with the best of it, I'm making sure that I'm maintaining as much of the best of it as I can before the flop by driving out drawing hands and simplifying my decisions on the flop.

Getting back to straights, if you raise UTG with KQu, the button calls and the flop comes JhThX, it's more important to lead or re-raise the flop OOP than it is IP, because you want to take down the pot ASAP or see whether or not the opponent is on the superior draw. AXh or 54h etc. is in the buttons range there, and assuming the opponent is on the same or similar straight draw is wishful thinking. If you are on a draw, then you always have to consider whether or not the opponent is on a better draw, i.e straight vs flush, ignorant straight vs nut straight, baby flush vs big flush, big flush vs nut flush etc.

The principle works in the reverse to, if some one opens from the button against the blinds, their inclined to do it with hands as low as AXs, 20s or suited connectors etc. If you had flat called with KQ here, for whatever reason, then betting or check raising this flop becomes important, because it immediately tells you whether or not your up against high cards or a flush draw etc.

Basically, when your opponent is in position or in a multi-way pot with 2 to a flush on the board, you're either folding or playing aggressively to push out the better draw with your straight or determine whether or not your outs are good or bad. Even when your'e in position with the straight draw H2H, you want to play more aggressively, because 2 suited high cards aren't uncommon either.

I don't really know how else to explain it, if it doesn't make sense now, you'll figure it out around 3/6 or 5/10 when the LAGGs show up and you have to play thinking aggressive instead of tight aggressive. You can see it at the lowest limits to tho', where fish just limp in and create a "school" and the sharks come in with AXs behind them, and it's really common in any form of limit poker.

Edit: Replace odds with likely hood, I didn't mean to imply that some is mathematically more likely to be dealt a suited hand when he's in position, I meant some one is more inclined to play a hand because it's suited when he has position. You wouldn't necessarily open JTs UTG, but you'd probably call or open with JTs if you were on the button. Some hands are more situationally or positionally bound than others in ABC poker.