There isn't any 1 specific reason. More than half a dozen. Some of which may still be a little sensitive so I will only point out the obvious. You have to realize that it isn't purely a matter of if a weapon is useful but if it is useful enough to justify consumption of a spot onboard. Mines are effective but are almost never carried unless the mission will involve using them.

One thing was the shift to more of a vetical external tube approach to submarine launched missles. The sub harpoon was encapulated version of the same one everyone else used. To have used it in external launchers would have caused some major reengineering. The tomahawk was much more suited for that role.

Another thing was that it's active radar homing could be fooled with newer countermeasures systems. It was also poor at target discrimination. Especially when launched OTH from a sub. Launching it into a dense contact enviroment meant having no idea who it may choose to go after. Even potentially a friendly.

Next target acquisition. If not getting targeting data from another source (which required being at PD and having a radio antenna up then the sub would need to use an RDF system (AN-BRD7 in the day) to figure out firing parameters. This thing had a sizable antenna and you would need multiple intercepts from substantial distances apart (due to long range of target)

They were very restricted as far as speed/depth launch conditions. Meaning at launch anyone seeing it launch not only knew where you were but had a damned good idea what speed and depth.

In the end these and other factors weighed together to make the space more valuable when loaded with other ordinance. Not that having a Harpoon on board would be useless. This isn't the first/only weapon this has happened to. Just one of the few publicly known about.

The Harpoon Question
 

The Harpoon and TLSMs have differences. The Tomahawks have a much longer range and a much bigger warhead so they would be ideal to shoot when you recieve a radio message telling you to shoot at target coordinates that are out of your Harpoons range. Harpoon's are faster and more agile. This means that they are harder to shoot down. They also skim the waves whereas the Tomahawks fly fairly high in the air. I usually stock up on Mk 48 ADCAPs and Harpoons, but since i am not you, and you might have a different playing style then me, you might choose differently. One strategy that i would recomend is to stock up with 16 TLSMs and the rest Mk 48s on a Seawolf. (this is for the mission where you have to destroy a convoy). You go up to missile launching depth right after you detect the convoy. Launch all 16 of your TLSMs at the center of the convoy and then dive to max operating depth and go all ahead flank for about 15 miles. while you are racing aross the bottom of the ocean, all of your targets will be frantically trying to avoid your missiles. get within 15 miles of the convoy and then launch your Mk 48s in volleys of 8. Try to stay in front of the convoy. When you have launched your last volley, cut all wires and make a 90 degree turn left or right. then go to 20 knots (max silent speed). They wont be able to catch you while they have about 32 ADCAPS chasing them around. you should be a safe distance away by the time that your fish explode. by this time the f-16 falcons should be coming your way and cleaning up the remaining ships.

(this is a scenarion from subguru.com/dwmissions.htm)

What's a "TLSM"?

Thank you Rip for answering. I appreciate your insight. I kind of understand the rationale for the Harpoon's currently that are in service. But if I understand it correctly, Harpoon II's correct the engagement envelopes, countermeasures issues, and a number of other things. I'm sure there are reasons that you can't say, due to security issues. I myself avoid topics on weapons I have actually fired (Mavericks ;)) for the same reasons. But looking at it from someone who has never fired a Harpoon, and only knows about Harpoons from open sources, I do see some usefulness to them.

I'm certain I could justify their existence and reasonably argue for why they should be deployed on US SSN's. I'm certain I could argue for the inception of Block II's on Virginia's. It's a shame they removed the ability to even use them on this platform. I know there might have been people who might have argued this back and forth behind the closed doors of the US Navy (Need to know basis) brass. As a voter and taxpayer, I've got to trust they made the right decision. But if we ever have to face China in the Taiwan straits, I'm hoping we're going to be using all options including the newer Harpoons on our SSN's. I myself see them as just another tool for our submarine sailors to use in a set of tactical situations.

Is it called a TASM?
 

TASM or whatever theyre called, its the tomahawk anti ship missile in DW