I've been wondering why this particular model of power armor (the one you find in Fort Constantine) doesn't have the strength boost and agility penalty of other power armor. It gives you rad resistance, that makes sense, and a charisma boost when wearing the helmet. Why is that?

Isn't the point of power armor that it is powered and therefor gives you strength? And if it doesn't impose an agility penalty you could effectively sneak in it, which seems a bit silly.

What do you think?

The +1 CHR is likely because of the supposed venerable status of the suit's original intended wearers, much like the Enclave power helmet confers a -1 penalty due to the Enclave's maligned standing throughout the series.

The lack of a STR modifier was probably to balance the items abnormally high health and the fact it's the most protective armor in the game (and has no other negatives except the impact of heavy armor on the sneak skill). The lack of AGL penalty may be due to the fact that there's a visible "underarmor" which could allow for more freedom of movement. OR, it could just be Bethesda being Bethesda and not thinking things through.

As I recall, it is mentioned somewhere that the T-51B power armor has under armor, similar to how the T-45 armor has the "recon armor" under it.

The T-51B being more advanced means it probably has a better version of the recon armor under it, which would negate the speed loss.