Stacking with Quads

Not often do I felt 2 opponents when I have quads, but I did here:

Full Tilt Poker $0.25/$0.50 Pot Limit Hold'em - 8 players - http://www.thehandconverter.com/hands/294322
The Official DeucesCracked.com Hand History Converter

CO: $29.45
Hero (BTN): $63.25
SB: $11.75
BB: $24.80
UTG: $45.95
UTG+1: $31.40
MP1: $31.70
MP2: $25.00

Pre Flop: ($0.75) Hero is BTN with 6d 7d
UTG raises to $1.75, 2 folds, MP2 calls $1.75, CO calls $1.75, Hero calls $1.75, 2 folds
With all of the action behind, I call my suited connectors.

Flop: ($7.75) 5d 7h 7c (4 players)
UTG checks, MP2 bets $3.85, CO calls $3.85, Hero calls $3.85, UTG folds
MP2 cbets & CO calls - I do not have either of them on draws. With the flopped 2nd nuts, I'm going to draw them in & not give away my hand. Hopefully MP2 will play this one for me...

Turn: ($19.30) 7s (3 players)
MP2 bets $9.65, CO calls $9.65, Hero calls $9.65
Excellent! I'm not sure why he feels compelled to bet half pot here, but I think he's going for value. Clearly, neither of them put me on quads. Perhaps they are thinking either has a 5 or AK or AQ?

River: ($48.25) 3h (3 players)
MP2 bets $9.75 all in, CO calls $9.75, Hero raises to $19.50, CO calls $4.45 all in
Thanks, MP2. You have enabled me to stack the both of you. There was quite a bit of hesitation in the CO calling the last $4.45 - he knew he was beat the whole way.

Final Pot: $86.40
CO mucks Th Ts
Hero shows 6d 7d (four of a kind, Sevens)
MP2 mucks Td Tc
Hero wins $8.90
Hero wins $74.50
(Rake: $3.00)
7's full for both parties. I'm not quite sure why they stacked off here - the way I played this, I would have been scared if I were them. 2 streets I smooth on the end; I have to AT LEAST put them on a decent pair. They don't stop & think about the possibility of a better pocket pair? I could easily show up with JJ. I don't know that I'm going to stack off with an overpair on pretty much any board. I guess, though, they figure if I had a set of 5's on the flop, I'm counterfeited by the turn 7...?