There's no formal statement, but given the actor they changed to, it's pretty easy to see that they wanted a Richards who is older-feeling, and more charming. Kraskinski played Richards as fairly smirking and cynical, which is absolutely how Richards has been at times, but not in all portrayals. If they wanted a similar vibe, there are a lot of actors they could have turned to. But they turned to an actor who specializes in warm, charming guys many of whom are ultimately kind of jerks (putting it mildly). He also tends to play older than Krasinski (despite only being four years older).
Also if we're being really real, outside of comedic roles (which he seems to have largely abandoned), Krasinski is a perfectly okay actor, whereas Pedro Pascal is a genuinely good actor. There is quite a gulf in talent there. I don't want to make up numerical ratings or something buy Pascal is better at fading into the character and better at giving emotive performances.
On top of this I'm not sure if Krasinski would even want to be a potential long-term fixture in other people's projects, as it seems like he has quite a lot going on (esp. behind the camera). Plus you'd really feel like an idiot for casting him as Richards if you couldn't get Blunt as Sue Storm!
Nor would one use it about Tony Stark though, from the comics. Yet RDJ undeniably brought a lot of warmth and humanity to the role. I think you're going to have Richards as a potentially important and recurring character, especially if he at all is "the team's dad", you probably want him to be a little bit more likeable than the comics - still a jerk I hope but not "this guy totally sucks" which is I think is a common reaction to comics Reed Richards.
Every role I've seen her in previously she was kind of creepy or weird, so it's an interesting casting from that perspective. She's more of a Lady Macbeth kind of cast than you'd normally think of for Sue Storm but it definitely gets my attention, but maybe she's just going to show her range.
Ebon Moss-Bachrach just based on The Bear I think will be perfect for The Thing, and I'm not familiar with Joseph Quinn, though the lewks he serves are very messianic so he may be an interesting Johnny Storm.