The Democrats say it is convenient and the Colorado clerks say that
all-mail voting reduces the costs and complexity of elections overall. 
The media plays along.

The Denver Post: Those promoting the changes said the bill is uniquely Colorado, and the
state could take the lead nationally on making elections more convenient
to voters.

However, the media ignores the data,
reporting only the shiny side of story and not the looming negative
impact on voters.  Leading the nation in convenience?  The big loser will
be the voters of Colorado. Without even getting into the increased
potential of absentee fraud with mail ballots, the big secret being kept
from voters is that all-mail and absentee voting balloting is not the
preferential way of voting because it has a much higher rate of
rejection due to voter errors and other issues that ultimately rejects
the ballot.  And turnout will be reduced except in special and local
elections.

In 2012, the Scientific-American highlighted a report that exposed the weaknesses on mail voting:

A new report by the Voting Technology Project (VTP)—a
joint venture between the California Institute of Technology and the
Massachusetts Institute of Technology—finds that even though absentee
ballots account for about only a quarter of all ballots cast during an
election, the number of uncounted absentee and election-day ballots may
be roughly the same.

The researchers estimate that
up to 3.9 million absentee ballots were requested but not received by
voters in the 2008 presidential election. Another 2.9 million ballots
sent to voters requesting them were not
returned for counting. And 800,000 returned absentee ballots were
rejected for one reason or another. In all, 21 percent of requested
absentee ballots were never counted in 2008—35.5 million requests for
absentee ballots led to 27.9 million mail-in ballots being counted.

Studies in Florida showed more issues and, more importantly, a much higher rate of
rejection of absentee and absentee provisional ballots.  Additionally,
according to a University of California study (highlighted in article), “experimental evidence
finds that mail-in voting can actually lower turnout by about 2.6%,
except in special or less publicized elections, where it raises turnout
by 7.6%.”