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In the regions of a positive moment, a portion of the slab works with the beam (both under compression force), so that part should be included in the beam design. However, negative moments develop cracks at the upper part of the beam. So, the cracked part of the concrete section is neglected in the design, and the lower portion is in compression which is rectangular.
Bcz , there is a negative moment above the supports.Making it useless and only left is rectangular portion at the bottom, so that's why rectangular beams are used for support moments in continous beams.
While in the span case, the top section is under compression and below one is in tension so the flexural strength is enough to contribute to resist the stresses.
