Tractors under 75hp have easier requirements under the Tier 4 rules. Meeting those does not require EGR and DEF like on pickup trucks and semis which have stricter limits on a per HP basis. Under 25.5hp it's easier still, and DPF or similar is not needed as it can all be done with engine tuning.
DPF has not been forced on the manufacturers, they are free to use whatever solution meets the requirements. Emissions regs are usually made with input from manufacturers- it does no good to make a reg that no one can meet.
There are four different ways that different manufactuers use to meet < 75hp Tier 4:
DPF, EFI and regens.
DPF+DOC, mechanical FI and continual low temp regen
DOC+EGR+EFI, no regens.
DPF, mechanical FI and regens
DOC is Diesel Oxidation Catalyst. It burns soot in a different way than a DPF.
Most manufacturers use #1. Branson uses #2. Mahindra and some Masseys use #3. Kubota's
B3350 used #4. They all have plusses and minuses. With #1 and #2 (and #4), operating the tractor in a way that keeps the DPF hot helps prevent problems. Not leaving it idling for long periods and working it periodically seems to reduce problems and extend time between regens. That might help type 3 too, I don't know. If you use a tractor to do 10 minutes of loader work every once in a while in a northern winter you'll have to deal with more regens and potential issues per hour than someone who is mostly doing long sessions of mowing in the summer.
A few manufacturers have had problems with their emissions system. Most notably the Kubota
B3350, which used a unique system that didn't work well especially in cold weather and is apparently being redesigned.