The story is in the data
The data shows that Brexit was a major factor in voter's decision at the ballot box.
Labour’s messaging on Brexit was muddled from the start. Unlike the Conservatives, that had a clear message of “get Brexit done”, Labour was indecisive and only tacked towards remain at the eleventh hour. Whenever Labour has won election victories in the past (think 1997, 1966 and 1945), it has been able to deliver a clear message and tell a compelling story. In 2019, Labour had no story or message.
How could Labour hope to deliver a clear message when is leader would not reveal how he would campaign in the event of a second Brexit referendum? The 2019 general election was touted the Brexit election. For Labour to not have a clearly defined position, and with its leader seemingly at odds with his party and the supporters over the issue, it was setting itself up for failure. When Corbyn did finally reveal his stance, two weeks into the 2019 election campaign, he vowed to stay neutral in the event of a second referendum. In doing so, he curtailed his party’s ability to become a leading voice on the single biggest issue in the 2019 election, leaving a path for the Conservatives to hammer home their message and emerge as the most decisive party on Brexit.  In the run-up to the election, YouGov polls consistently showed that the British public had a much clearer understanding of the Conservatives’ Brexit policy. Labour could have delivered a far clearer message by taking a clear stance after the European elections in May.
Labour’s Brexit message was exceptionally clear. It was essentially the sole goal of the People’s Vote campaign. Labour vowed to hold a second referendum with the option to remain and the terms of a government-negotiated Brexit agreement. The issue wasn’t that Labour didn’t have a clear policy, it was that the hostile British media deliberately muddied the waters for deliberately partisan ends. 
[P1] Labour had no clear message on Brexit. [P2] Brexit was the defining issue of the 2019 election] [P3] Therefore, Labour had no clear message on the defining issue of the election. [P4] This cost them a lot of votes.
[Rejecting P1] Labour had a very clear message on Brexit.