Shall we say Rav Shayshes assumes it is because of Paysach - נימא קא סבר רב ששת משום פסח
Summary:
Something which was prohibited remains prohibited even if the original reason is no longer valid. ריב"ב, validated a ק"פ which was offered in the morning, only בדיעבד, however לכתחלה it is forbidden to bring the ק"פ until the afternoon.
All of them also require reclining - כולהו נמי צריכי הסיבה
Summary:
תוספות is unsure if one is required to drink again if he did not recline, and/or if he is permitted to drink again if he did not recline by the third cup.