Rough Roman Memes

Well, that's also the thing - taxes in the most vicious wars of the mid-Republic were very 'ad hoc'. For the mid-Republic, it was something to the tune of "Wow, we need money, tell everyone in the city that we're taking their earrings to sell for the Survival Of The Republic", while gathering taxes across an entire Empire without losing 90% of it to corruption and general lawlessness is much more difficult - and, considering the very makeshift tax collection standards of the Empire, difficult to scale up. It has to be more 'structured' to ensure that any of the money gets to the central government, and the Empire was generally hesitant about imposing new standardized taxes outside of the city of Rome itself.

Rather than sources of recruits being concentrated, you'd also have to deal with Roman coloniae being scattered throughout the Empire, meaning gathering even just a handful of men from each one would mean a whole lot of shipping new recruits to where they needed to go - bearable when there's no immediate crisis, difficult when you need a bunch of soldiers to replenish the ranks at once.

That being said, there was considerably more money available for the Legions - it was, in fact, by far the greatest expense of the Roman government. But as mentioned, the Legions were also considerably more expensive - getting all the supply depots and guarded wagon trains to reach every frontier of the Empire is no cheap task!