Or HaChaim on Genesis 6:18/אור החיים על בראשית ו׳:י״ח

1 והקימותי את בריתי. I shall fulfil My covenant. Why did G'd not first tell Noach to enter the ark seeing that his survival depended on that? Our sages (Bereshit Rabbah 31,12) say that the covenant in question was merely that Noach's peers would not kill him when he would enter the ark. In that event it is quite appropriate that the Torah first mentioned G'd as maintaining His covenant with Noach.

2An additional meaning maybe that G'd was afraid that Noach would not consider his salvation as a permanent one. He might feel that what was about to occur now might occur again some generations later if mankind did not improve. G'd therefore already hinted to him what He would spell out in more detail after Noach's thanks-giving offering in 8,21. In the meantime Noach had to enter the ark to escape the destruction that would kill all flesh.

3G'd may also have wanted to reassure Noach prior to his entering the ark that He would not abandon him during the time it took to sit out the deluge in the ark. Noach should not worry that the ark itself would prove a death trap. This assurance helped Noach as we know from 8,1 where the Torah mentions that G'd remembered Noach during all that time. After this G'd commanded Noach to enter the ark under the protection of this covenant. Although the covenant is not mentioned again in connection with 7,1, G'd relied on His previous assurance to Noach in this regard. The covenant is alluded to again in 8,1 where G'd is reported as "remembering" all those in the ark. Even though Rashi understands G'd as "remembering" Noach's good deeds and the animals refraining from mating while in the ark, this is homiletics and not the plain meaning of the verse.