Twitter users have always had the urge to poll random things and since there was no poll feature at the time, they had to improvise by using the retweet and favourite (now like button). That ruse went on for a while and although it was rather effective, it was not an official way to create polls on Twitter.
The company might have noticed the activity with the random polls being posted on the network and that is why they formally announced the polling functionality on the last quarter of 2015. Initially, it was limited to 2 polling items and expired within a day but that got an update where now you can add an additional 2 items to a poll and can be set to last a week. This feature has been so popular in that Twitter actually revealed that people posted 1.7 billion votes within 3 months!
In order for you to create a poll on mobile or on the desktop, you had to tap on the pie chart icon which was adjacent to the recent GIF button. Its choice was rather straight forward: people associate the pie chart with a poll since it is one of the visual aids used to show an outcome of a poll. However, the company has decided to change this pie chart poll icon for one for a horizontal bar graph one and it totally makes sense.
If you have used the polling feature, the live results or the final one are displayed in the horizontal bar format since it goes well with the format of the timeline. Having the pie chart as an icon for the polling function which displays results as horizontal bars is a mismatch in its own right and that is why probably the company decided to change it. The company didn’t change anything else in the polling functionality so this was just a subtle but important cosmetic change.