The reason is simple.
Teams that dont have world class flanks, are easely locked up with the 3-5-2.
Its a simple domminant formation, that just needs adaptation in case of 4-2-3-1
neutralizing, locking up opponents etc those are the weaklings' defensive mentality in any kind of competition.
you can go all defensive against muhammad ali for 12 rounds neutralizing all his punches yet the jury will hand the win to ali if you can't even scratch him.
obsessing with such mentality just show how a loser you are.
imposing your play > neutralizing your opponents.
we did that in the 1st 6 months of conte's reign, so why we haven't done it against since? it's definitely conte's fault with his own mentality and approach.
and no, blunders we had at the start of this season also conte's fault, it's his job to prepare for the most part the same team he already had past 2.5 years, to suddenly found his players blundering right and left meant something went wrong.
the lack of wingers excuse can't hold because we still have such players, there's also nothing that prevent conte from using 4-3-2-1, we have quite many flexible players.
another thing is the way conte set up vidal marchisio CM always be so far ahead of the pitch (and also these so called defensive wingers but placed far ahead on the pitch while still laughably unable to give any significant offensive threat?!?) easily exposing 3 CBs and one regista for counter attacks, the exact same thing I've seen since the conversion to 3-5-2, too easily read if the opponent did little study to this team, and some did, many times we conceded in such similar way it's just not funny, such glaring weakness should be addressed long ago and simple tactical changes should've been made, but conte didn't do that.