The thing is, tanks that require a high velocity cannon must sacrifice on HE filler for their HE rounds. It's very much a one or the other sort of deal. Think back to the Sherman, you have a choice between the 75 or the 76, or the short barreled 75 vs the long barreled 75 on the Pz IV, to achieve higher velocity you can't have as high HE filler.

Sure you can put howitzers on tanks, like the 105 on the Sherman, but your options are limited to shaped charge weapons instead of kinetic kill. That's a serious problem today as a tank's primary anti armor shell today is the APDSFS or whatever acronym, the pointy one. That's why every single tank gun on the face of the planet today are of the high velocity type.

So you have the major downside of much lower HE filler, plus the fact it is a tank. You can't elevate that much, you don't have indirect fire sights, and the big problem is the sheer cost of the thing. Self propelled artillery does not need tank level armor or tank level tactical mobility. There's absolutely zero reason why you'd ever use a tank for indirect fires for the simple reason they aren't sighted for indirect fire. And why would be, they have such limited range and do a worse job for a higher price, so why bother in the first place?