Why doesn’t my red robin tree grow anymore?

Hi all,

About 4 or 5 years ago I did a major cutback on this mature red robin. I used to have to trim it regularly, but since then I haven’t needed to. It’s pretty much been like the picture year round with no growth at all.

The leaves on it make me think I haven’t killed it…but clearly have done something wrong/bad. Any advice?

Thanks,