Why do some people not perform the Tarawih prayer on the the first and last day of Ramadan?

Tarawih prayer is a Sunnah, and there is established encouragement for it in the words of the Prophet, peace and blessings be upon him: "Whoever prays during the nights of Ramadan with faith and expecting Allah's reward, all his past sins will be forgiven." There is no evidence for the desirability of abandoning it on the first or last night; rather, the Hadith encourages praying throughout all of Ramadan. Whoever neglects praying during Ramadan misses out on a great reward, but there is no sin upon him, because abandoning Tarawih is neither forbidden nor obligatory.