Why must sin come from much speaking?
Rambam (on Avos 1:17): Most speech is excess and sin. Surely he will say at least one thing improper to say. 'A fence for Chachmah is silence' (Avos 3:13). Much speaking is a sign of a Kesil - "v'Kol Kesil b'Rov Devarim" (Koheles 5:2). Minimizing words shows one's high level and good lineage - 'those of proper lineage are silent' (when two argue, the first to stop speaking has better lineage - Kidushin 71b).
R. Yonah: Man's heart cannot coordinate and weigh his words. It is proper to evaluate one's Da'as before speaking. One cannot speak much [properly] without stopping to think in between, to answer his colleague's words.
Mesilas Yesharim (Perek 13): If speech is not forbidden, the Torah permits it. However, it leads to much Bitul Torah, Leshon ha'Ra, Sheker and frivolity.
Malbim: Most divisions of speech, even permitted, bring to sin.
Why is "Choshech Sefasav Maskil"?
R. Yonah: Sechel is investigation, via which one comes to Tevunah. One who investigates to know things that need constant thought, he does not speak until he finishes investigation. However, a Navon, who accustomed himself to investigate with his Sechel, and reached Tevunah in most matters that are found, from what he investigated, he contemplates what is similar without great investigation; a Navon understands a matter amidst another matter. Therefore, he does not always cease his lips [from speaking]. "Otem Sefasav Navon" (17:28) is a different matter (refer to 17:28:2:1).
Orchos Tzadikim (Sha'ar ha'Shetikah DH Amru): Hashem did not command to be silent like a deafmute. Rather, he must refrain from speaking about his colleague.
Malbim: The Rambam (Avos 1:17) says that there are four kinds of speech - forbidden, optional, Mitzvah, and excess. A wise person speaks only Mitzvah. Most speech brings to sin. A wise person holds back his lips, and speaks only words of Chachmah.
