פּירוש אויף דברים 16:14

ושמחת בחגך, “you shall rejoice on your festival;” “rejoicing” has been mentioned already in connection with the festival of weeks (verse 11); The reason why it was mentioned there is that it is the period of harvesting a major part of the grain harvest. (Isaiah 9,2.) At the time of Sukkot, such joy is increased greatly as everything that the fields and orchards produce is now being gathered in. Joy is not mentioned in connection with Passover as the season of ingathering produce has not yet started. Furthermore” we have been taught in the Mishnah Rosh hashanah 16,1 that one of the four days on which the world is being judged concerning the produce for the coming year is Passover. In other words, the fate of the harvest still hangs in the balance, and it would be premature to rejoice therefore.

והגר והיתום והאלמנה, they will rejoice over finding the various gleanings of the tree fruit harvested at that period.

ושמחת בחגך, “you shall be joyous on your festival.” Our sages in Moed Katan 8 state that from the words “be joyous on your festival” one can infer that one should not get married on the intermediate days of the festival, as that would be mixing two kinds of joy together, something not appropriate. The Torah mentions here that the joy it has in mind is the joy with G’d’s blessing, for it adds: “for the Lord your G’d has blessed you with all your harvest,” etc. You will observe that the expression שמחה does not occur in connection with the Pessach festival. Neither is mention made of the word ברכה, blessing, in connection with that festival. The reason is that on the other two festivals different parts of the harvest are already at hand, i.e. G’d’s blessing has become manifest, something not yet apparent on Pessach. Seeing that on the other festivals the harvest is in the home it is possible to share it with the four categories of people mentioned in the Torah. This adds to the joy of the festival.
On Pessach, not only has the new harvest not yet been brought in but the old harvest is just about depleted. It is therefore difficult to provide joy for the people with whom the Torah wants us to share our joy. By not demanding joy from us on Pessach, the Torah displayed a deep psychological insight into the minds of most of us, who, even though we might share with the Levite, etc. even at that time, would not do so joyously.
Another reason why joy has not been mentioned by the Torah in connection with Pessach though it does appear in connection with Shavuot, and on Sukkot the Torah mentions joy no fewer than three times, is in line with the statement in Rosh Hashanah 16 that the world is judged on four occasions during the year, On Pessach G’d decides if the harvest (grain) will be bountiful; on Shavuot G’d decides if the harvest of the orchards will be bountiful; on Sukkot G’d decides on the water supply for the coming year, whereas on Rosh Hashanah G’d judges the fates of every individual for the forthcoming year. Seeing that Pessach is not only a day of judgment, but we do not know yet how this judgment will manifest itself seeing the year is still ahead, it is not yet time for rejoicing (Pessikta d'Rav Kahane 29). By the time Shavuot occurs it has already become clear that the barley and wheat have matured, etc. There is therefore ample reason for joy. By the time Sukkot comes around the other three judgments have also been seen to have been favourable; therefore the Torah mentions joy three times in connection with that festival.