There is no CURRENT reason not to. However, the RNAV (GPS) RWY 30 was split into a Y and a Z version in 2010. 2010 was near the beginning of the whole "Climb Gradient for a lower DA/MDA" idea, and criteria was changing quickly as the rules settled down. I can't remember specifically if we couldn't have two LNAV minimums on the same chart at the time, but it wouldn't surprise me too much as there were a lot of policy letters and emails around that time about the whole CG issue.
Today, though, yes they could be (and should be) combined on the same chart.
Incidentally, check out what's on the FAA's IFP Gateway, Production Plan tab for SQL. The "Z" is being canceled, and (presumably) the two are being combined into one, as there will be no suffix letter on the other one anymore.
View attachment 112122
I've addressed it before, but it seems like a good time to mention it again - although each procedure is reviewed every two years, they are only reviewed from the standpoint of "is this procedure safe?" Primarily this is an obstacle evaluation, to see if any new obstacles cause a problem. In addition, Flight Inspection flies the procedures regularly for the same reason (and to check other aspects of the airport infrastructure like lighting).But that's it - the current procedure is evaluated/flown, and if it's still safe, it's checked off. There is no attempt to evaluate the procedure to see if it is optimized for the current rules, or current airspace, or maybe it could be better designed today, or anything like that. So existing procedures, as long as they are safe, will stay published under whatever criteria they were originally established under, possibly for decades*. Note that a "minor amendment" where the amendment letter is updated (e.g. from Amdt 1 to 1A or 3C to 3D, etc.) has very limited changes that are allowed, so such an amendment does not allow for major redesigns.
UNLESS there is user or ATC input - THAT is what drives changes to existing procedures. The primary conduit for those changes is through the airport manager to the FAA, but anyone can submit comments through the IFP Gateway.
* A client was asking me just the other day about "why" something on a SID was the way it was - I looked at the chart and at the bottom it had the last amendment date - 1987! Yes, it's been checked every 2 years, found to be safe, nobody is requesting changes, so it stays as-is.