There is a multitude of reasons for this. Beyond some limitations which still limit the ipad, it is the chick and an egg problem. You won't have real software on the iPad until there are users (buyers) for it and you won't have users until the software exists. This is especially important for corporate usage - at my work place I haven't heard of any iPads being provided to the employees, which means, no one can use an iPad and buy/develop software for one.
There are and were limitations preventing desktop class software on the iPad, some have lifted but it also takes quite a long time to take up on that, see also above.
- Keyboard support. For a lot of tasks, you need a keyboard. It took a long time until Apple sold keyboards for the iPad, and it still was quirky. There is a row of keys missing and even when coupling bluetooth keyboards, the extra keys didn't work. What Apple didn't really get was: a lot of people need an Esc key for their work, many would like funktion keys. It is still a struggle to run as simple as an ssh session on the iPad. Also, so far I haven't found a truely good SSH client on the iPad.
- Mouse support - really has been added only with 13.4, we will see how this develops.
Restrictions by Apple:
- For a long time, there was no real file exchange between apps, no joint file spaces. This has been partially fixed, still feels cumbersome and very much depends on the apps to get right.
- There is a ton of stuff that could be done on the iPad, but is banned outright. Termux is great on Android, not allowed on the iPad. The iPad would be great for development, if not mostly disallowed.
- And yes, for a lot of things, the screen is just too small :).
- The app store. Great and bad at the same time. The issues are probably best seen on macOS, because on macOS the app store is not mandatory. A lost of applications only exists outside the app store. While I liked the convenience of the app store and the fact that I don't have to deal with license keys and was free to use the app I bought across all my devices, the perpetual updates for free are condemning most longterm development. Paid upgrades were the life blood of the software industry for decades. They mean a regular revenue stream from power users like subscriptions, but didn't force the casual user to constantly pay, only when that user wanted the features of the newer versions. Subscriptions currently seem to be the only way, how a actively developed application can survive in that universe, but limits the user base.
- In app purchases. In principle a good idea, it lead to a reality where apps are flooding the app store which constantly try to extort money from the user. That games are allowed to charge in total more than lets say 2x what an AAA title would cost, is scandaleous. A model where an app could offer a free trial and then is purchased normally would be much more supportive of quality app development.
Many of this can be worked around, but unfortunately that is rather the exception. There are still great apps around which are a joy to use, but the whole ecosystem is drowned by all the bad stuff.