Because you're going to have to wait a year or two to add them to your lineup. By all accounts, Habs want to start competing next year and need the offence ASAP.
Also, you have a much better idea of what you have in Zegras than you do in a 7-10 forward.
Do we need offence ASAP though? What's likely to hold us back from competing next year is actually likely to be the D. Our offence has for the most part been ok since our top line started to produce. If you then add Dach and Newhook who missed most of the past 20+ games plus potentially even more development for Slaf and maybe a guy like Roy or Heineman adding depth scoring over the likes of Armia then offence doesn't seem like it would be a major issue.
What has been an issue has been the GAs, during that same period we were 4th worst in the NHL, and are 6th worst over the whole season. And next year probably isn't going to be better, Savard might get traded, we'll potentially be integrating more rookies since Hutson, Mailloux, and Reinbacher will all be pushing for NHL time.
Getting help for the top-6 will be nice to do, and certainly something we have to do if we want to be contenders. But talking competing for the playoffs it's just not likely to be the biggest hurdle we face next year. So waiting 1-2 years for that top-6 F is actually ideal for us since that gives our young D the time/experience they need to come into their own and gives us the best cap efficiency for when we need it the most since Slaf, Dach, Guhle, and probably Xhekaj will all be getting paid top dollars so having an ELC come in then is the perfect moment.
Also having a much better idea of what you have in Zegras compared to Eiserman also hurts Zegras because some of that much better idea is bad.
... Celebrate time
