The Garden was exceptionally well designed and executed at the time, given the extraordinarily challenging constraints. Its facades are blank and boring mainly because it was almost completely surrounded when built (IIRC, there were some areas with zero exterior workspace). Much of the exterior blockwork was done from the inside out:
^Boston Globe archive
It's capacity, while not in the biggest tier, is
not small at all by NBA/NHL standards (it's solidly mid-pack). And its internal design/layout remains highly functional and well thought-out. This is what effective design under extreme constraints looks like. Exterior aesthetics can always be updated later; some of the major arenas have had significant exterior refreshes at their 20-30 year marks anyway (e.g. Target Center in Minneapolis). It didn't make sense to do that here before knowing what the new surrounding buildings plans were going to be anyhow.