The founder of your religion, and writer of your canon books was an ardent polygamist. And that's less than 170 years ago...hardly a blink in religious history.
135 years without polygamy isn't a very long time at all.
Small "o" orthodox Christians however have for 2,000 years frowned on polygamy...forbidding it expressly for Church leadership in the Bible (I Tim. 3:2; Titus 1:6) and historically forbidding it even for lay persons from the 4th Century onwards....meaning Smith could not have held any leadership post in any Church--nor even membership there for the last 1600 years or so. How can one breaking the New Testament command on polygamy--and firmly teaching others do do so too--be considered a true prophet?
It's actually less than 120 years since the edict came down. Also, remember that polygamous families weren't banned from the church; just the official act itself. So LDS polygamous families were still widely intact well into the 1900s--less than a century ago. I mean there's an awful lot of baby boomers whose grandparents were born within polygamous families...so that's not so long ago.
Many others have written about "less official" LDS polygamous marriage rites that were conducted on a case by case basis, though not publicly condoned, in the 1890s and on into the 20th century.