That's because (I==77) is a 2D binary image (true/false) but when you pass a binary/logical image into an array in MATLAB, you get a column vector out. What you want to do is to mask the image:
mask = (I==77); C% Create binary (logical) mask.
maskedImage = I .* uint8(mask); % Multiply mask by image.
imshow(maskedImage); % Display maskedImage