Why does gpg --genkey fail for su user with error "gpg cancelled by user gpg Key generation canceled." automatically on Red Hat Enterprise Linux 7 whereas works for direct login?

Issue

• Why does gpg --genkey fail for su user with error "gpg cancelled by user gpg Key generation canceled." automatically on Red Hat Enterprise Linux 7 whereas works for direct login?
• The strace shows error as follows.

9891  20:21:48 read(5, "INQUIRE PINENTRY_LAUNCHED 9980\n", 1002) = 31 <0.009746>
9891  20:21:48 write(5, "END", 3)       = 3 <0.000018>
9891  20:21:48 write(5, "\n", 1)        = 1 <0.000013>
9891  20:21:48 read(5, "ERR 83886179 Operation cancelled <Pinentry>\n", 1002) = 44 <0.251577>
9891  20:21:49 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f485fd8d000 <0.000019>
9891  20:21:49 write(2, "gpg: cancelled by user\n", 23) = 23 <0.000204>
9891  20:21:49 write(2, "gpg: Key generation canceled.\n", 30) = 30 <0.000074>
9891  20:21:49 open("/home/testuser/.gnupg/random_seed", O_WRONLY|O_CREAT, 0600) = 6 <0.000039>
9891  20:21:49 fcntl(6, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0 <0.000027>
9891  20:21:49 ftruncate(6, 0)          = 0 <0.000149>