Temp HP and normal HP behave differently, so lumping them in together could be misleading and confusing. For instance, if you are at full health, then take 5 damage, and then are given 5 temp HP, you aren't at full health. If you combine temp HP and current HP displays, it would look like you were at 30/30, but that's not quite right. Temp HP and normal HP are separate pools: normal HP can be healed until you reach your max (so, in the situation above, if you got healed for 5, you would be at 35/30), but temp HP can never be healed or even added to - only replaced by other temp HP.
Like many things in this game, temp HP needs to be explained better. It's not the most simple or intuitive mechanism and I believe it is currently being lumped in incorrectly with HP Max increases from Aid and shielding from Arcane Ward (i.e. you should be able to have all three at the same time, but they're all being coded as temp HP, which is wrong). For the most part, I'm a fan of sticking with the 5e rules as written, but if there's a system that could benefit from a little streamlining, this is it - it just needs to be explained well.
