Out-turtling him gives him the advantage, is the reason. You’re giving him a situation where he controls the match completely and you have to guess an unsafe move to deal damage.
Letting Sim have his midrange spot sucks because you can’t sonic boom, you can’t jump at him (because Sim’s antiairs beat or trade with your jump-ins), and you can’t just sit there because Sim will probably end up chipping you with a couple fireballs. You can’t keep Sim across the screen, because he just has too many ways to get over or under sonic booms and into midrange, and you can’t just hold down-back and wait for when you think he’ll throw a limb so you can flash kick because it’s an unsafe guess that’s unlikely to work at a reasonable rate. If you do flash kick him, great, it’s just unlikely and unsafe.
Guile’s best options against Sim are after a blocked boom or boom-neutralized fireball and up close. Getting Sim to block a boom or neutralize a boom with a fireball means that Guile either gets frame advantage or gets de facto frame advantage because his moves start up faster and have better priority at certain ranges. The only real way to get Sim to block/neutralize a boom or to get up close is to throw a boom and walk up/toward+forward/jump jab/etc behind it, and then once that happens to do some high-priority move that gets you back to where you can safely throw a boom, that is to play hit and run, running back to get a boom out, following it up, and running out again. If you ever get a knockdown, go in for a crossup, but if he blocks the crossup, start up your games again. That said, this is a losing matchup because it’s not hard for Sim to get into his sweet spot than it is for you to successfully deal damage while hitting and running.
Honda v Guile is pretty different from Guile v Sim, by the way. Each of those characters has a different thing he wants to do: Honda wants to get in on Guile; Guile wants to keep Honda out; Sim wants to stand midrange on Guile; Guile wants to play hit and run on Sim.