Sometimes this is because they were repeatedly drilled early in their bridge careers "always bid a major first", but I've also noticed that some folks will shy away from a 2C call because the auction 1D - 2C always seems to go wrong for them.
Let me say this (hopefully) once and for all:
On game-forcing hands, always start with your longest suit.
Do I need to repeat that?
Always, always, always bid your longest suit first when responding to partner if you're going to force to game. Millions of good slams have been missed when people respond with their Jxxx suit rather than their AQJTx suit. Let's stop this epidemic right now!
If you're one of those folks who hate the auction 1D - 2C, I have a nice toy for you. Eric Kokish came up with this great rebid structure (passed along to me by my old fishing buddy Greg).
In the uncontested auction 1D - 2C:
- 2D = any unbalanced hand with 5+ diamonds, may have a 4-card major
- 2H = 4=4=4=1 (singleton club)
- 2S = good club raise (no 4-card major)
- 2NT = 12-14 balanced
- 3C = bad club raise (no 4-card major)
- 3D = six or more good diamonds, 16+ HCP
- 3H = splinter raise of clubs
- 3S = splinter raise of clubs
- 3NT = 18-19 balanced
The real joy in this response structure is opener's having two ways to raise clubs. You'll find out at the three level whether slam is a possibility, and have lots of room to explore.