Probably because it allows them to canonise a design that although originated in a game has featured in various official/licenced merchandise over the years, but also provides a clean slate so they aren’t beholden to using it for any potential future 25th century show.
But they're doing it in a very strange way. According to
Star Trek Online the
Enterprise-F was launched in 2409, not 2386. The
Enterprise-E is still in service in
STO until 2408. All the evidence points to
Picard season three taking place in 2405ish so the E should still be going.
If they wanted to canonise the
Odyssey-class, just have the USS
Odyssey show up. This canonises an
STO design at the expense of blowing all the surrounding continuity to hell and back. What's the point?
To reflect the passage of time while paying service (fan service) to the Enterprise's history. Using the E would have led to the questions of why wasn't Picard and crew onboard her this whole time?
Why? Picard and crew didn't go their separate ways when the E launched. Kirk and crew didn't go their separate ways when the A launched. We saw Riker, Troi, Crusher, and Data already depart the
Enterprise-E in
Nemesis. The back story to
Picard's first season had Picard himself leave the ship when he was promoted to admiral to manage the Romulus crisis. Geordi could have left the
Enterprise to work on the Jellyfish with Ambassador Spock in the 2380s as part of Picard's Romulus task force. What Worf was doing on the
Enterprise in
Nemesis was never confirmed anyway, maybe he took a year-long sabbatical from being Federation ambassador to the court of Chancellor Martok and then resumed his post. It's all easily explained using existing information from the shows and movies. And let's face it, the fact that the youngest of the TNG characters is now pushing 70 is good enough a reason for them to not still be on the
Enterprise anyway.