回归结果不现实标准差和t值-经管之家官网！

求问各位，我在用stata做tobit回归的时候，回归结果只报告了系数，后面的标准差等一概没有显示，请问这是怎么回事。
回归指令和结果粘贴如下。
. tobit y x x1 female rural spouse family_income capital age age2 nok medical child_6 old educating hea2 hea3 income* edu* inter* cont* prov* if x>0,ll(0)
note: income6 omitted because of collinearity
note: edu1 omitted because of collinearity
note: edu6 omitted because of collinearity
note: educating omitted because of collinearity
note: inter6 omitted because of collinearity
note: cont6 omitted because of collinearity
note: prov28 omitted because of collinearity
Tobit regression Number of obs = 689
LR chi2(-2) = 164.34
Prob > chi2 = .
Log likelihood = -2780.7055 Pseudo R2 = 0.0287
-------------------------------------------------------------------------------
y | Coef. Std. Err. t P>|t| [95% Conf. Interval]
--------------+----------------------------------------------------------------
x |-.6169301 . . . . .
x1 |-1.544358 . . . . .
female | 308.5881 . . . . .
rural | 2399.79 . . . . .
spouse | 8172.818 . . . . .
family_income |-.0400585 . . . . .
capital | .0052382 . . . . .
age |-135.3925 . . . . .
age2 | 1.154023 . . . . .
nok | 964.334 . . . . .
medical | .3042272 . . . . .
child_6 |-1340.845 . . . . .
old | 562.0484 . . . . .
educating | 1348.314 . . . . .
hea2 |-3179.851 . . . . .
hea3 |-5728.797 . . . . .
income | 15064.45 . . . . .
income1 | 81487.27 . . . . .
income2 | 43647.54 . . . . .
income3 | 28901.11 . . . . .
income4 | 20139.66 . . . . .
income5 | 6943.817 . . . . .
income6 | 0(omitted)
education | 4710.893 . . . . .
edu1 | 0(omitted)
edu2 | 34828.54 . . . . .
edu3 | 21299.57 . . . . .
edu4 | 15109.28 . . . . .
edu5 | 8632.847 . . . . .
edu6 | 0(omitted)
educating | 0(omitted)
inter |-6202.899 . . . . .
inter1 |-47596.26 . . . . .
inter2 |-37115.42 . . . . .
inter3 |-30175.22 . . . . .
inter4 |-14823.49 . . . . .
inter5 |-170.6715 . . . . .
inter6 | 0(omitted)
cont1 |-24.34978 . . . . .
cont2 | 2174.874 . . . . .
cont3 | 3067.975 . . . . .
cont4 | 17295.52 . . . . .
cont5 |-126623.5 . . . . .
cont6 | 0(omitted)
prov1 | 132143.7 . . . . .
prov2 | 162208.3 . . . . .
prov3 | 160395.8 . . . . .
prov4 | 136128.8 . . . . .
prov5 | 128610.7 . . . . .
prov6 | 130331.1 . . . . .
prov7 | 123069.6 . . . . .
prov8 | -11763.7 . . . . .
prov9 | 142927.6 . . . . .
prov10 | 132067.1 . . . . .
prov11 | 152299.7 . . . . .
prov12 | 139295.9 . . . . .
prov13 | 138108.8 . . . . .
prov14 | 157024 . . . . .
prov15 | 127413.9 . . . . .
prov16 | 144719.1 . . . . .
prov17 | 161653 . . . . .
prov18 | 120934.4 . . . . .
prov19 | 138549.8 . . . . .
prov20 | 138531.9 . . . . .
prov21 | 141810.1 . . . . .
prov22 | 128652.3 . . . . .
prov23 | 128737.5 . . . . .
prov24 | 122557.4 . . . . .
prov25 | 132126.2 . . . . .
prov26 | 132608.1 . . . . .
prov27 | 138292.7 . . . . .
prov28 | 0(omitted)
_cons |-257824.5 . . . . .
--------------+----------------------------------------------------------------
/sigma | 22998.69 . . .
-------------------------------------------------------------------------------
Obs. summary: 461left-censored observations at transfer_i~e<=0
228 uncensored observations
0 right-censored observations
.
当我把表示地区因素的一系列变量prov*删除之后，结果又能正常显示了。请问这到底是怎么回事呢？