calculation and analysis – The matrix calculation does not go to the end, why?

The one-off product is not calculated until the end, why?

$ p = 2; n = 2; j = 2; $
$ Y = text {Table} left[left{y_kright},{k,1,n}right]; $
$ B = text {Table} left[left{beta_kright},{k,1,p}right]; $
$ xi = text {Table} left[left{epsilon_kright},{k,1,n}right]; $
$ X = text {Table} left[x_{k,v},{k,1,n},{v,1,j}right]; $

$ text {RSS} = (Y-X.B) ^ T. (Y-X.B); $
$ text {Solve}[nabla_{text{Flatten}[B]} text {RSS} = 0, text {Flatten}[B]]…. (1) $
$ left { left { beta _1 to – frac {y_1x_ {2,2} -y_2 x_ {1,2}} {x_ {1,2} x_ {2,1} -x_ {1 , 1} x_ {2,2}}, beta_2 to – frac {y_2 x_ {1,1} -y_1x_ {2,1}} {x_ {1,2} x_ {2,1} -x_ { 1,1} x_ {2,2}} right } right $
$ H = X. Left (X ^ T.X right) ^ {- 1} .X ^ T $

Simplify[X.Flatten@(B /. (1)) == Dot[H, Y] ]

$ left {y_1, y_2 right } = left (
begin {array} {cc}
ten \
0 & 1 \
end {array}
right left(
begin {array} {c}
y_1 \
y_2 \
end {array}
right) $