Why did the people not listen to Moshe?
Rashi: They could not be comforted on account of the shortness of breath (the result of their troubles) and the heaviness of work. 1
Ramban: They did not pay attention to Moshe's words for fear that Pharaoh would have them killed by the sword 2 and on the account of the ongoing pressure of the taskmasters.
Rashbam: Even though they believed Moshe when he first appeared to them, 3 because they understood that they would be given a respite from the heavy work, and instead, their workload had increased.
Seforno: They did not stop to reflect, to have faith in Hashem's salvation, 4 that Hashem should acknowledge their faithfulness and consider it a kindness on their part. 5 This was the result of a lack of faith in Hashem and the hard work. 6
Targum Yonasan: They did not listen to Moshe due to their fussiness and because they worshipped idols.
Hadar Zekenim: It was hard for them to separate themselves from the idolatry they served - "Mishchu u'Kchu Lachem" - withdraw your hands from idolatry.
Hadar Zekenim: This is difficult. Anyone who hears good news is happy!
See 4:31 (Rashbam).
And that explains why Hashem's statement "And I will give it to them as an inheritance" was not fulfilled with them, only with their children (Seforno).
If not for which they would have taken Moshe's words to heart and realized that the correct thing to do was to place their trust in Hashem. And it is due to the lack of faith that the promise "ve'Nasati lachem Morashah" was not fulfilled with them, only with their children (Seforno).